Introduction to the estuary, its management and monitoring

The Derwent Estuary lies at the heart of the Hobart metropolitan area and was a major route for Tasmanian Aboriginals between the coast and-hinterland for around 40,000 years. The Derwent is also a waterway of great natural beauty and diversity. It is an integral part of Tasmania’s cultural, economic and natural heritage as well as an important and productive ecosystem that supports a wide range of habitats and species.


The Tasmanian Aboriginal Centre’s (TAC) Language Program has undertaken both linguistics and historical research on the original languages of Tasmania to retrieve and revive Aboriginal language in Tasmania — palawa kani. The name of the River Derwent in palawa kani, is timtumili minanya. Previously, the Aboriginal names for the River Derwent came from G.A. Robinson’s records from the 1830s, where he attempted to give his idea of their sound by dividing words into syllables. The names for the Derwent were noted as: TEETOOMELE MENENNYE, RAY. GHE.PY.ER.REN.NE and NIB.BER.LIN.

Today, approximately 40% of Tasmania’s population — 227,000 people — live around the estuary’s margins. The Derwent is widely used for swimming, boating, fishing, marine transport and industry. Further upstream, the River Derwent supplies about 60% of the region’s drinking water and is a major source of hydroelectric power.

A number of environmental issues affect the Derwent Estuary, in particular:

  • heavy metal contamination
  • poor recreational water quality at some bays and beaches
  • low oxygen levels in the upper estuary during summer
  • elevated nutrient concentrations
  • environmental flows and barriers
  • introduced marine pests and weeds
  • loss of habitats and species
  • impacts of climate change, e.g. sea level rise, erosion and habitat loss

Although there have been significant improvements in the treatment of sewage and industrial wastes over the past decade, the Derwent still faces a number of environmental challenges. A strategic and coordinated planning approach across all levels of government, industry and the community is our best hope for a clean and healthy estuary in the future.

Murphys Flat wetland in the upper Derwent Estuary. Image: Derwent Estuary Program.
Murphys Flat wetland in the upper Derwent Estuary.

Image: Derwent Estuary Program

Environmental monitoring and reporting

A fundamental requirement for effective natural resource management is an on-going and reliable source of environmental data. This principle forms the basis of the DEP’s cooperative monitoring program between the state government, councils, industries and research institutes. Formerly independent monitoring programs are now coordinated to provide better information on the estuary as a whole, and to report annually on environmental conditions and trends in the Derwent.

This ‘Report Card’ summarises monitoring data collected by the DEP and our partners, as well as other relevant information collected during , and early , including:

  • weekly recreational water quality testing during summer months
  • monthly whole-of-estuary water quality monitoring
  • surveys of heavy metal levels in fish and shellfish
  • biological surveys (seagrass, spotted handfish, little penguins)
  • weeds survey and erosion monitoring

More detailed information is published in our five-yearly State of the Derwent Estuary reports, available on our website.

Management and restoration

The Derwent Estuary Program (DEP) was established in 1999 as a partnership to share science for the benefit of the community, nature and the economy. The program has been successful in bringing together a wide range of stakeholders — firstly to build a common understanding, vision and management framework — and secondly to progressively implement this vision through partnership agreements and practical action.

The program was initially designed to address environmental quality issues such as industrial and urban water pollution, contaminated sediments, invasive species and loss of estuarine ecosystems. More recently, our scope has broadened to include the catchment influences, as well as education and amenity. Key program areas include environmental monitoring and reporting, coordination of regional activities, stormwater management, heavy metal investigations, wetland, saltmarsh and seagrass restoration, and promotion of walking tracks.

Our program partners

The DEP, a not-for-profit company limited by guarantee, is supported by the Tasmanian Government, six councils that border on the estuary (Brighton, Clarence, Derwent Valley, Glenorchy, Hobart and Kingborough Councils), five business partners (Nyrstar Hobart, Norske Skog Boyer, TasWater, TasPorts and Hydro Tasmania), NRM South, EPA Tasmania and research partner Institute for Marine and Antarctic Studies, University of Tasmania. Other project supporters include the CSIRO, The Ian Potter Foundation and EcoDetection.

Seagrass survey. Image: Derwent Estuary Program.
Seagrass survey.

Image: Derwent Estuary Program

Tasman Bridge, Hobart.
Tasman Bridge, Hobart.

Image: Derwent Estuary Program

View of Hobart city from kunanyi / Mount Wellington. Image: Tourism Tasmania / Rob Burnett.

Image: Tourism Tasmania / Rob Burnett

Sampling sites and discharge points

The DEP Beach Watch and ambient water quality monitoring sites are located on the map along the location of wastewater treatment plants and our industry partners Nyrstar and Norske Skog Boyer.

Derwent estuary map kunanyi / Mount Wellington HOBART New Norfolk Bridgewater Austins Ferry Old Beach Claremont Glenorchy Moonah New Town Lindisfarne Bellerive Howrah Tranmere Rokeby Lauderdale South Arm Sandy Bay Taroona Kingston Blackmans Bay Tinderbox Bridgewater Causeway Bowen Bridge Tasman Bridge Iron Pot North West Bay Ralphs Bay Ambient monitoring site (temperature, salinity, pH, dissolved oxygen, nutrients, chlorophyll a, metals) Bellerive Beach (east) Bellerive Beach (west) Blackmans Bay Beach (mid) Blackmans Bay Beach (north) Blackmans Bay Beach (south) Hinsby Beach Howrah Beach (east) Howrah Beach (mid) Howrah Beach (west) Kingston Beach (mid) Kingston Beach (north) Kingston Beach (south) Little Howrah Beach Little Sandy Bay (north) Little Sandy Bay (south) Nutgrove Beach (east) Nutgrove Beach (west) Taroona Beach Windermere Bay Beach Brooke Street Pier Brown’s River Elwick Bay Geilston Bay Hobart Rivulet Kangaroo Bay Lindisfarne Bay Marieville Esplanade Mid-River Derwent Swim Montagu Bay New Norfolk (The Esplanade) New Norfolk (Millbrook Rise Jetty) New Town Bay Old Beach, Jetty Road Prince of Wales Bay Marina Hobart Regatta Pavilion Sullivans Cove Victoria Dock Waterman’s Dock Brighton Brighton East Risdon East Risdon New Norfolk New Norfolk Bridgewater Bridgewater Cameron Bay Cameron Bay Rosny Rosny Rokeby Rokeby Blackmans Bay Blackmans Bay Prince of Wales Bay Prince of Wales Bay Macquarie Point Macquarie Point Sandy Bay (outfall from Selfs Point) Sandy Bay (outfall from Selfs Point) Selfs Point (outfall at Sandy Bay) Selfs Point (outfall at Sandy Bay) Norske Skog Paper Norske Skog Paper Nyrstar Hobart Smelter Nyrstar Hobart Smelter Water quality monitoring sites in 2022 Beach and bay monitoring sites (enterococci) Good water quality Fair water quality Poor water quality N/A – long-term rating not available Ambient monitoring sites (temperature, salinity, pH, dissolved oxygen, nutrients, chlorophyll a, metals) Sewage treatment plant discharges in 2022 < 1000 ML/yr 1000–2000 ML/yr > 2000 ML/yr Industrial discharges in 2022 > 50,000 kL/d EXPLORE MAP EXPLORE MAP

Use the “Explore map” button to find the Beach Watch and ambient water quality monitoring sites, the location and discharge rates of waste water treatment plants and large industry. The layers on the map can be viewed separately or together. Or view a list of the locations in the tables that follow.

Beach Watch water quality monitoring sites in 2022
Location Long-term rating
Bellerive Beach (east) N/A
Bellerive Beach (west) Good
Blackmans Bay Beach (mid) Fair
Blackmans Bay Beach (north) N/A
Blackmans Bay Beach (south) N/A
Hinsby Beach Fair
Howrah Beach (east) Fair
Howrah Beach (mid) Fair
Howrah Beach (west) Fair
Kingston Beach (mid) Fair
Kingston Beach (north) Poor
Kingston Beach (south) N/A
Little Howrah Beach Good
Little Sandy Bay (north) Good
Little Sandy Bay (south) Good
Nutgrove Beach (east) Fair
Nutgrove Beach (west) Fair
Taroona Beach Fair
Windermere Bay Beach Good
Bay Watch water quality monitoring sites in 2022
Location Long-term rating
Brooke Street Pier Good
Brown’s River Poor
Elwick Bay Good
Geilston Bay Poor
Hobart Rivulet Poor
Kangaroo Bay Good
Lindisfarne Bay Poor
Marieville Esplanade Poor
Mid-River Derwent Swim Good
Montagu Bay Good
New Norfolk (Esplanade) Fair
New Norfolk (Millbrook Rise Jetty) N/A
New Town Bay Poor
Old Beach, Jetty Road Good
Prince of Wales Bay Marina Fair
Hobart Regatta Pavilion Poor
Sullivans Cove Good
Victoria Dock Poor
Waterman’s Dock Poor
Sewage treatment plant discharges
Location Result
Brighton < 1000 ML/yr
East Risdon < 1000 ML/yr
New Norfolk < 1000 ML/yr
Bridgewater < 1000 ML/yr
Rosny < 1000 ML/yr
Rokeby < 1000 ML/yr
Cameron Bay 1000–2000 ML/yr
Blackmans Bay 1000–2000 ML/yr
Prince of Wales Bay > 2000 ML/yr
Macquarie Point > 2000 ML/yr
Sandy Bay (outfall from Selfs Point) > 2000 ML/yr
Industrial discharges
Location Result
Norske Skog Paper > 50,000 kL/d
Nyrstar Hobart Smelter > 50,000 kL/d

Derwent estuary catchment zones

From the river to the sea the River Derwent and estuary supports a variety of habitats, species and human uses which are depicted in the following five illustrations. Environmental inputs that affect these areas are also highlighted.

Pollution sources, loads and trends

Pollutants of particular concern in the Derwent Estuary include:

  • heavy metals, as these may be toxic to aquatic plants and animals and accumulate in seafood — a potential health risk for local anglers
  • excessive nutrients, as these can trigger algal blooms that reduce water clarity, smother fish habitat and deplete oxygen — low oxygen may result in fish kills, rotten egg odours and release of nutrients and heavy metals from sediments
  • pathogens from human sewage that are a human health risk
  • sediments, as these reduce light available to aquatic plants
  • litter — particularly floating plastics

Contaminant loads summary

What comes from where?

  • The River Derwent catchment is the primary source of total nitrogen and total suspended solids to the Derwent Estuary.
  • TasWater sewage treatment plants are the primary source of dissolved nutrients and phosphorus, and the majority of the load is discharged to the mid and lower estuary. In the 2022 financial year, dissolved inorganic nitrogen loads increased significantly.
  • Contaminated groundwater is the primary source of zinc to the estuary.
Nutrients: total nitrogen (tonnes/year)
1800 1600 1400 1200 1000 800 600 400 200 0 Tonnes 2017–18 2018–19 2019–20 2020–21 2021–22 River 797.0437 Stormwater 167 Sewage 443.8434 Industry 134.8985 River 778.2648 Stormwater 167 Sewage 392.7028 Industry 126.1135 River 1006.673 Stormwater 167 Sewage 336.7871 Industry 120.4035 River 715.9454 Stormwater 167 Sewage 360.061 Industry 142.9435 River 970.4159 Stormwater 167 Sewage 481.2437 Industry 130.6135 Industry Sewage Stormwater River
Nutrients: dissolved inorganic nitrogen (tonnes/year)
800 700 600 500 400 300 200 100 0 Tonnes 2017–18 2018–19 2019–20 2020–21 2021–22 River 138 Stormwater 49 Sewage 361 Industry 32.6 River 143 Stormwater 49 Sewage 309 Industry 32.6 River 194 Stormwater 49 Sewage 282 Industry 39.4 River 122 Stormwater 49 Sewage 293 Industry 32.77 River 170 Stormwater 49 Sewage 399 Industry 39.52 Industry Sewage Stormwater River
Nutrients: total phosphorus (tonnes/year)
200 175 150 125 100 75 50 25 0 Tonnes 2017–18 2018–19 2019–20 2020–21 2021–22 River 36.96581 Stormwater 30 Sewage 78.91475 Industry 12.185 River 36.00136 Stormwater 30 Sewage 69.97209 Industry 12.39 River 49.81729 Stormwater 30 Sewage 67.04447 Industry 14.5 River 33.92778 Stormwater 30 Sewage 63.90397 Industry 16.65 River 43.11017 Stormwater 30 Sewage 79.65882 Industry 13.24 Industry Sewage Stormwater River
Zinc (tonnes/year)
160 140 120 100 80 60 40 20 0 Tonnes 2017–18 2018–19 2019–20 2020–21 2021–22 Nyrstar groundwater 120 Nyrstar operations 1.239513985 Stormwater 7 Sewage 1.2 Nyrstar groundwater 120 Nyrstar operations 1.923317106 Stormwater 7 Sewage 1.2 Nyrstar groundwater 120 Nyrstar operations 2.720312911 Stormwater 7 Sewage 1.2 Nyrstar groundwater 120 Nyrstar operations 3.089824839 Stormwater 7 Sewage 1.2 Nyrstar groundwater 80 Nyrstar operations 2.183777005 Stormwater 7 Sewage 1.2 Sewage Stormwater Nyrstar operations Nyrstar groundwater
 Nyrstar GroundwaterNyrstar operationsStormwaterSewage
Sediments as total suspended solids (tonnes/year)
40,000 35,000 30,000 25,000 20,000 15,000 10,000 5,000 0 Tonnes 2017–18 2018–19 2019–20 2020–21 2021–22 River 13366.09 Stormwater 7996 Sewage 225.9979 Industry 847.8 River 15594.04 Stormwater 7996 Sewage 194.9803 Industry 716.1 River 30820.34 Stormwater 7996 Sewage 168.6332 Industry 534.0691 River 8378.973 Stormwater 7996 Sewage 227.479 Industry 1223.876 River 23294.85 Stormwater 7996 Sewage 234.0561 Industry 959.43 Industry Sewage Stormwater River
Organic matter as biochemical oxygen demand (tonnes/year)
1000 800 600 400 200 0 Tonnes 2017–18 2018–19 2019–20 2020–21 2021–22 Sewage 468.9 Norske Skog Boyer 331.3 Sewage 418.2 Norske Skog Boyer 345.5 Sewage 338.2 Norske Skog Boyer 324.5 Sewage 442.1 Norske Skog Boyer 454.8 Sewage 636.0 Norske Skog Boyer 239.6 Norske Skog Boyer Sewage
 SewageNorske Skog Boyer

Pollution sources

Pollution enters the Derwent Estuary from many sources, commonly referred to as ‘point sources’ and ‘diffuse sources’.

Point sources include sewage treatment plants and large industries, such as the Norske Skog paper mill at Boyer and Nyrstar Hobart zinc smelter at Lutana.

Diffuse sources include stormwater runoff from urban areas as well as the larger catchment inputs carried by the Derwent and Jordan rivers. Other diffuse pollutant sources include air pollution, landfills, aquaculture operations, and wastes associated with ports and marinas. Sediments within the estuary itself may also release pollutants into the overlying waters under certain conditions.

See the point sources map for more details.

River Derwent catchment

The River Derwent is a significant source of contaminants to the estuary. Previous monitoring has indicated that major nutrient sources of nutrients in the catchment are agriculture (in the winter) and aquaculture (in the summer). Early warning signs of nutrient stress including growth of benthic blue-green algae leading to taste and odour issues in Hobart’s main water supply as well as filamentous algal blooms smothering valuable macrophyte communities have raised concerns about the water quality of the River Derwent catchment upstream of New Norfolk.

Loads from the River Derwent catchment are typically variable, however in recent years dissolved nutrient loads have been elevated. River Derwent loads are relatively coarse and are provided as an indicative value of the contribution of contaminant loads to the estuary from the catchment (above Meadowbank). An in situ nutrient analyser project funded by The Ian Potter Foundation has been established in the catchment to better understand nutrient sources, calculate loads and understand nutrient dynamics. An overview of this project is provided in section 5.5.

Sewage treatment plants

Sewage treatment plants (STPs) are the largest source of bioavailable nutrients (nutrients available to living things such as algae), followed by the catchment, stormwater and the Norske Skog paper mill. The 2022 financial year was characterised by an increase in loads discharged to the estuary from point sources, particularly STPs.

Effluent reuse turns a waste product into valuable, nutrient-enriched irrigation water, removing nutrients that would otherwise enter the Derwent Estuary. The last two financial years have seen a significant decline in reuse due to a combination of high rainfall and compromised infrastructure. At the Greenpoint STP (Bridgewater) increased discharge volume and secondary treatment bypass due to heavy rainfall resulted in increased nutrients discharged to the estuary. During prolonged rainfall events, private water storages reach capacity and demand for recycled water decreases resulting in greater discharge.

TasWater reuse (megalitres)
4000 3500 3000 2500 2000 1500 1000 500 0 Megalitres 2016 2017 2018 2019 2020 Selfs Point 0 Rosny 1299.8 Rokeby 748.9 Cameron Bay 119 Bridgewater 785 Selfs Point 24 Rosny 1626.3 Rokeby 799.4 Cameron Bay 132.3 Bridgewater 660.6 Selfs Point 24 Rosny 1618.2 Rokeby 837 Cameron Bay 98.3 Bridgewater 851.2 Selfs Point 22 Rosny 974.1 Rokeby 828.1 Cameron Bay 81.4 Bridgewater 522.4 Selfs Point 24 Rosny 377.3 Rokeby 965.8 Cameron Bay 60 Bridgewater 522.8 Bridgewater Cameron Bay Rokeby Rosny Selfs Point
 BridgewaterCameron BayRokebyRosnySelfs Point

Norske Skog

Key contaminants originating from the site affecting estuarine water quality are suspended solids, resin acids, organic matter and dissolved nutrients. The commissioning of the biological secondary effluent treatment plant (SETP) in 2008 has been very effective in reducing suspended solids, biological oxygen demand and resin acids. However, increased nutrient and suspended solid discharge was evident in the Norske Skog Boyer combined effluent stream in 2022. This noticeable increase was due to filamentous bacteria outbreaks in the biological secondary treatment plant (SETP) throughout 2021. To better assess and respond to nutrients in the secondary treatment plant a real-time water quality analyser will be installed in 2023 to measure nitrogen including ammonium.


Industries have historically been the main source of heavy metal pollution to the Derwent, however loads have declined significantly in recent years. Contaminated groundwater at Nyrstar is the largest source and is being captured and treated using a series of innovative projects. In the 2022 financial year, zinc extracted from groundwater increased by 50% (120t) due to the recent upgrades of the groundwater extraction system.

Nyrstar: trace metals extracted from groundwater (tonnes)
120 100 80 60 40 20 0 Tonnes 2017–18 2018–19 2019–20 2020–21 2021–22 Zinc 82.8 Cadmium 2.5 Zinc 89.0 Cadmium 2.2 Zinc 82.5 Cadmium 1.7 Zinc 74.0 Cadmium 1.5 Zinc 116.5 Cadmium 2.5 Cadmium Zinc

Did you know?

There has been a 40% reduction in nutrients reaching the lower estuary due to the 2019 upgrade of the Blackmans Bay wastewater treatment plant (TasWater).

The Derwent Estuary Program is a partnership between state and local government and industry to restore and promote the Derwent. We share science for the benefit of nature, the economy and the community.

Kelp forest. Image: IMAS.

Image: IMAS

Derwent water quality

The DEP ambient and recreational water quality monitoring programs have been operating for 23 years and provide a basis for assessments relating to swimming at beaches, and a whole-of-estuary health check. Current emphasis includes keeping tabs on dissolved oxygen levels and nutrients in the upper estuary and assessing nutrients in the catchment using real-time water quality analysers.

Climatic conditions

Annual rainfall for the financial year (731 mm) was above the long-term average (613mm; all records since 1893) as recorded at the Bureau of Meteorology (BOM) Hobart weather station (Ellerslie Road) (Fig. 5.1). Total spring rainfall (322mm) was the fourth highest on record, well above the long-term average (168.8mm). Summer rainfall (December–March inclusive) during the 2021–22 season (131 mm) was below average (186mm).

Rainfall at Ellerslie Road, Hobart, Bureau of Meteorology
160 140 120 100 80 60 40 20 0 Rainfall (mm) Month (July 2021 – June 2022) Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul 2021 14.2mm Jul 129 year average 51.6mm Aug 2021 50.6mm Aug 129 year average 54.3mm Sep 2021 71.6mm Sep 129 year average 52.9mm Oct 2021 160.4mm Oct 129 year average 62.1mm Nov 2021 90mm Nov 129 year average 54.3mm Dec 2021 40.8mm Dec 129 year average 56.4mm Jan 2022 54.4mm Jan 129 year average 46.7mm Feb 2022 5.4mm Feb 129 year average 39.2mm Mar 2022 30.6mm Mar 129 year average 44.6mm Apr 2022 21mm Apr 129 year average 49.6mm May 2022 134.6mm May 129 year average 47.6mm Jun 2022 58.2mm Jun 129 year average 53.8mm 2021/2022 rainfall 129 year average
Month2021/2022 rainfall (mm)129 year average (mm)
Sunset at Nutgrove Beach, Sandy Bay.
Sunset at Nutgrove Beach, Sandy Bay.

Image: Derwent Estuary Program

Black swans.
Black swans.

Image: Derwent Estuary Program

Red seaweed.
Red seaweed.

Image: PWS

Swimming in the Derwent

Each summer recreational water quality is monitored at about 38 beaches and bays around the estuary through a collaborative State and Local Government program. Sampling is conducted weekly from December through March at the locations shown in our sampling and discharge points map.

To describe the risk level to swimmers a colour coded system is used based on five years of monitoring data: green indicates good, yellow indicates fair, and red indicates poor water quality.

During the 2022–23 Recreational Water Quality (RWQ) season, the water quality at almost all of our 19 swimming sites was significantly better than the last monitoring season, with less than half the number of failures, which occurs when the enterococci results exceed the prescribed trigger level of 140 MPN 100 mL⁻¹ set by the Tasmanian Recreational Water Quality Guidelines 2007 (DoH, 2007), thus requiring retesting. This season saw 22 exceedances, compared with 49 last summer. At the end of this season, five swimming sites were graded as Good, nine sites graded as Fair, one as Poor, with four sites yet to be classified. The sites that improved their rating were Bellerive Beach (west), from Fair to Good, and Howrah Beach (mid), from Poor to Fair. The only site to drop its rating was Hinsby Beach, from Good to Fair. Little Sandy Bay Beach continues to have the consistently best water quality in the program, and Kingston Beach (north) is currently the only site with a Poor rating.

The water quality at the 19 environmental sites was similar to the previous season. On 56 occasions, enterococci results over 140 MPN 100 mL⁻¹ were recorded, compared to 51 times last year. There are now seven environmental sites with Good long-term ratings, two as Fair, and nine as Poor. Two sites dropped their ratings, i.e. Prince of Wales Bay (from Good to Fair) and Victoria Dock (from Fair to Poor). No sites improved their rating following this summer’s sampling. The Mid-river Derwent sampling location continues to be the environmental site with the consistently best water quality, followed by Montagu Bay and now Elwick Bay.

Overall, it was a dry summer for western Tasmania with closer to average rainfall in the east, including in the Derwent Estuary where the first half of December was very wet followed by almost no rain until late January and largely only two rain periods in February and March. While rainfall is a common driver of pollution at our swimming sites, it is difficult to draw definite conclusions between rainfall and enterococci results. Many of this summer’s environmental site failures appear linked with rain, whereas the swimming site failures probably less so.

Collecting beach water samples.
Collecting beach water samples.

Image: Derwent Estuary Program

Taking a water sample from the beach.
Taking a water sample from the beach.

Image: H. Bobbi, DOH

Water quality indicators

The DEP coordinates a whole-of-estuary monitoring program that integrates sampling carried out by the DEP and EPA, Nyrstar Hobart, Norske Skog and TasWater. Water quality is monitored each month at 31 sites for indicators such as temperature, salinity, dissolved oxygen, suspended solids, nutrients, organic carbon, chlorophyll a and heavy metals. This information is used to document conditions and trends over time and to provide data for estuarine modelling and process studies.


Nutrients are essential for plant and algal growth and function. Plants and algae readily take up nutrients from their environment and their growth can be limited by low nutrient availability. A number of water quality problems are caused by elevated nutrients, particularly the excessive growth of algae. Excessive algal growth does occur at times in the Derwent Estuary and can have adverse effects including blooms of phytoplankton (microscopic marine algae), excessive growth of filamentous algae which may smother seagrass, excessive seaweed growth which can alter native habitats, low oxygen levels when algae die off which can kill fish and diminished aesthetic appeal due to foul odours and discolouration of the water.

The major sources of nutrients to the Derwent include the ocean, River Derwent catchment, STPs, the Boyer paper mill and urban run-off (see contaminant load section). These inputs vary from year to year and throughout the Derwent Estuary.

Seasonal and geographical patterns

Strong seasonal patterns are evident for dissolved nutrients in the Derwent. River Derwent and marine inputs are greatest in winter when high rainfall transports nutrients from the catchment to the estuary, and nutrient rich sub-antarctic waters intrude into the lower estuary. In summer, nutrient levels in surface waters are typically lower. These seasonal patterns are augmented by year-round inputs from sewerage treatment plants and industry sources which result in elevated nutrient levels, particularly in the mid-estuary.

General geographical patterns for nutrients in the Derwent Estuary include:

  • Nitrogen and phosphorus values increase downstream towards the estuary mouth;
  • Nutrient levels are greater at depth than in surface waters, except for the upper estuary; and
  • Dissolved nutrient concentrations are highest in the mid-estuary and upper-estuary bottom waters.

Changes over time

In the upper Derwent Estuary dissolved nutrient levels have increased significantly over the last 15 years. These increases have generally coincided with increased loads from local point sources in recent years (Norske Skog Boyer, Turriff Lodge STP and Green Point STP) and increased River Derwent catchment loads. This increase has also coincided with signals of nutrient stress in the upper estuary in 2015 including taste and odour issues in the Hobart drinking water supply, algal blooms smothering seagrass meadows, and fish kills (juvenile barracouta) due to low dissolved oxygen. To better understand nutrient supply and dynamics from the Derwent River catchment, the DEP commenced a three year project to trial in situ analysers at seven locations in the Derwent River catchment (see River Derwent Catchment – Water Quality).

In the mid-estuary where nutrients are typically greatest, dissolved nutrients have declined significantly. These declines have been observed throughout the mid-estuary and are related to significant improvements of local STP processes, particularly at Prince of Wales Bay (POWB) (Fig. 5.2; Fig. 5.3). However, nutrient levels in the mid-estuary increased in 2022, likely as a result of increased loads discharged from Derwent STPs (see contaminant load section for more information).

Monthly total ammonia nitrogen (TAN) concentrations (µg/L) at Prince of Wales Bay monitoring site
Monthly total ammonia nitrogen (TAN) concentrations (µg/L) at mid-estuary sites Prince of Wales Bay
Annual total ammonia nitrogen load (TAN) load discharged from Prince of Wales Bay STP (tonnes)
100 80 60 40 20 0 Tonnes 2014–15 2015–16 2016–17 2017–18 2018–19 2019–20 2020–21 2021–22 72.754 62.21177 60.44939 52.03288 23.65901 19.92484 46.27411 52.93725
 Total ammonia nitrogen – tonnes

River Derwent catchment – water quality

Monitoring water quality can be challenging as programs are costly and do not capture incidents, variable pollutant sources, rainfall-runoff events and in-stream processes such as nutrient uptake. To address these challenges, we are trialling real-time water quality analysers in the Derwent River catchment.

Funding from The Ian Potter Foundation, Hydro Tasmania, TasWater, the Environment Protection Authority and Meadowbank Vineyard enabled us to purchase six Eco Detection real-time water quality monitoring systems to measure nitrate, nitrite, phosphate, chloride, carbonate, sulphate and fluoride, as well as other key parameters. The analysers work remotely and in near real-time, automatically taking measurements every 1–6 hours. Data is sent to the Cloud where it can be accessed from anywhere on mobile devices or computers.

The technology was developed by Professor Michael Breadmore’s team at the University of Tasmania and engineered for commercialisation by Melbourne-based company Eco Detection, with the goal to provide robust, autonomous and low cost systems that provide data in real-time allowing fast and smarter decisions regarding industry operations and waterway management.

Map of real time analyser sites in the River Derwent catchment. Image: Derwent Estuary Program.
Map of real time analyser sites in the River Derwent catchment.
Installation of the first analyser at Turriff Lodge Wastewater Treatment Plant. Image: Derwent Estuary Program.
Installation of the first analyser at Turriff Lodge Wastewater Treatment Plant.

Image: Derwent Estuary Program

Since May 2022, all six Eco Detection units have been operating in the River Derwent catchment. The six sites include two outfall locations (TasWater’s sewage treatment plant Turriff Lodge at New Norfolk, and Norske Skog outfall), as well as three tributaries (Clyde, Ouse, and Florentine rivers), and the River Derwent below Meadowbank Dam. We have also inherited an additional Eco Detection unit from the University of Tasmania located at Westerway (Tyenna river). This is the first water quality monitoring network on a catchment scale using this technology.

To validate the data generated by the analysers, traditional grab samples have been collected as part of a 12 monthly assessment with nutrients analysed at Analytical Services Tasmania. The laboratory results are compared to data produced by the in-situ Eco Detection units to ensure accuracy. Once the analyser validation is complete (June 2023), we will investigate nutrient loads at each site. With this high frequency data, we also gain an understanding of daily nutrient variability and rainfall-runoff dynamics, which is not possible with traditional monthly monitoring programs. Figure 5.4 shows an example for high frequency nitrate data at the Clyde (Hamilton), in relation to flow.

Example of high frequency nitrate concentrations (orange, in ppb) measured at the River Clyde at Hamilton. Nitrate concentrations are diluted by an increase in flow (blue, in m³/s) in August 2022 and remain low during higher flow conditions, with the exception of a rainfall event in December 2022 causing nitrate concentrations to increase due to surface water runoff.
Example of high frequency nitrate concentrations (orange, in ppb)  measured at the River Clyde at Hamilton. Nitrate concentrations are diluted by an increase in flow (blue, in m³/s) in August 2022 and remain low during higher flow conditions, with the exception of a rainfall event in December 2022 causing nitrate concentrations to increase due to surface water runoff.

Estuary metal contamination

Levels of heavy metals in Derwent Estuary sediments are among the highest in Australia. Derwent sediments tend to be fine-grained and organic-rich and significantly exceed national sediment quality guidelines for zinc, copper, mercury, lead, cadmium and arsenic.

Ambient water quality data show a decline in heavy metal levels at most monitoring sites in the estuary, aligning with the most recent survey of metals in estuary sediments.

Please see State of the Derwent Estuary – 2020 Update for more information.

Lauderdale saltmarsh, Lauderdale, Tasmania. Image: Derwent Estuary Program.

Image: Derwent Estuary Program

Habitats and species

Important habitats of the Derwent include rocky reefs, saltmarshes and wetlands. The Derwent is also home to huge numbers of plants and animals. Two of note are the spotted handfish and the little penguin and their conservation is supported by the DEP in cooperation with local councils, scientists, State Government and the community. Human health is a key reporting priority for the DEP, particularly heavy metal levels in fish and shellfish.

Estuarine habitat and species

Surveys of the Derwent Estuary indicate that unvegetated, soft-bottom habitats are by far the most abundant habitats in the estuary (86%), followed by seagrass and macrophytes (7%; primarily in the upper estuary), tidal sandflats (6%; primarily in Ralphs Bay) and rocky reefs (1%; primarily in the lower estuary).

Little penguins – art burrows

Coordinated by Tasmanian artist Jane Bamford, this innovative artistic design project involved six professional artists, a photographer, and two emerging artists in a collaborative partnership with scientists and researchers from IMAS, Birdlife Tasmania and the Derwent Estuary Penguin Advisory Group (PAG). The intention was that artists design and create ceramic penguin nesting modules that incorporate design features noted in the researcher’s brief.

Exhibited modules were offered for sale to ‘gift into habitat’ to support little penguins’ nesting habitat throughout the Derwent Estuary. The Derwent Estuary Program purchased one of these nests with the generous donations from the Tasmanian Field Naturalists Club and the Derwent Valley Horse Riders Association.

All the modules were successfully stress tested in summer using small ibuttons to collect information about temperature and humidity. All the ceramic nests performed well, outperforming a design still in use and stood up to our current preferred box design used in the Derwent.

Little penguin art burrows. Image: Peter Whyte.
Little penguin art burrows.

Image: Peter Whyte

In 2022, the ceramic nests were taken to a colony by boat. Initially, they were not open for little penguins while onsite temperature and humidity was assessed. Happily, the nests passed the stress test and are now available to little penguins with monitors recently discovering penguin activity at one of the ceramic nests.

This project was made possible by the Australian Government’s Regional Arts Fund, which supports the arts in regional and remote Australia.

Visit Jane Bamford’s website.

Endangered bitterns found in the Derwent Estuary

The Australasian Bittern (Botaurus poiciloptilus) is one of the most cryptic birds that have their home in the upper estuary wetlands. It is listed as Endangered under the federal Environment Protection and Biodiversity Conservation Act 1999, and globally on the International Union for Conservation of Nature Red List.

Australasian Bittern. Image: Richard Hall.
Australasian Bittern.

Image: Richard Hall

With its camouflage-coloured plumage that blends in perfectly with background reedy vegetation, this bird is heard more often than it is seen. It has a very distinctive booming call that can be heard over large distances. Birds will freeze if approached, and on windy days may even sway to match the movement of the reeds, which adds to their elusive nature. They require large, relatively undisturbed wetlands, such as in the upper Derwent Estuary, where they breed in densely vegetated areas, building nests in deep cover over shallow water.

While formerly widespread in Tasmania, particularly in the east of the state, the number of bitterns and their range has contracted over the last twenty years. Actual numbers in the River Derwent are unknown, but thankfully they are still about. In 2022, DEP participated in The Bookend Trust’s NatureTrackers acoustics project, CallTrackers, where an acoustic recorder was deployed in the upper estuary wetlands near Dromedary for one week which picked up two distinctive booming calls of a male Australasian Bittern.

Heavy metals in seafood

Oysters and mussels from the Derwent contain high levels of heavy metals, particularly zinc, lead and cadmium. While levels have declined since 2003 including in the period between 2016–2020, in some areas (i.e. above the Tasman Bridge), they are still far in excess of national food standards. Mercury levels exceed national food standards in several species of Derwent caught fish — particularly black bream — and to a lesser degree flathead and trout.

In 2020, 67% of Derwent caught flathead had mercury concentrations exceeding the FSANZ maximum expected levels. 81% of flathead had concentrations exceeding the generally expected levels for zinc. Metal concentrations in fish caught from the Derwent were higher than those caught at the reference site (Mickey’s Bay near Bruny Island), and fish caught on the eastern shore and in Ralphs Bay tended to have higher concentrations of mercury in their flesh. This spatial trend has been previously observed and is consistent with our understanding of estuary circulation. Good news is that the 5-yearly rolling average for mercury is declining in flathead.

Sampling suggests that levels are lower in other recreationally targeted fish (e.g. whiting, Australian salmon, mullet, cod and flounder). Based on the most recent (2019–20) monitoring results for several recreationally caught seafood including flathead, oysters and mussels, there has been no change in current health advice, which is as follows:

  • Don’t eat shellfish collected from the Derwent (including Ralphs Bay).
  • Don’t eat any bream from the Derwent (including Browns River).
  • Limit consumption of other Derwent-caught fish to no more than two meals per week, or one meal per week for pregnant and breastfeeding women, women planning to become pregnant and young children.

For a full report see Metal Contamination in Fish and Shellfish of the Derwent Estuary 2020 which summarises all metals in fish data up to the summer of 2019–20.

Current health advice

  • DO NOT eat any bream from the Derwent (including Browns River)

    DO NOT eat any bream from the Derwent (including Browns River).

  • DO NOT eat shellfish collected from the Derwent (including Ralphs Bay)

    DO NOT eat shellfish collected from the Derwent (including Ralphs Bay).

  • LIMIT CONSUMPTION of other Derwent-caught fish

    LIMIT CONSUMPTION of other Derwent-caught fish to no more than two meals per week, or one meal per week for pregnant and breastfeeding women, women planning to become pregnant and young children.

Marine pests, weeds and disease

The Derwent Estuary is extensively colonised by introduced marine species. At least 79 invasive species have been recorded, including four species of particular national concern: Northern Pacific seastar, European green crab, Japanese seaweed, and European clam. A number of other species (e.g. New Zealand half crab, New Zealand seastar, and New Zealand screw shell) also pose a significant threat to the ecology of the estuary.

Rice grass (Spartina anglica) — a highly invasive intertidal weed — has been successfully managed in the Derwent Estuary through annual surveys and control actions. The area of infestation had been reduced from two hectares in 1995 to now possibly zero. No rice grass has been observed since 2016. Monitoring continues.

Northern Pacific seastars.
Northern Pacific seastars.

Image: Peter Mathew / TasWater


Image: Jon Sullivan

Karamu (Coprosma robusta) is an evergreen shrub originating from New Zealand that is a declared weed under the Tasmanian Weed Management Act 1999, requiring landholders to remove it from their property. The upper Derwent Estuary had previously been identified as the largest infestation in Tasmania. While much has been eradicated over the past five years, a large dense infestation remains around New Norfolk. Active and ongoing management of Karamu involves the Derwent Catchment Project, Department of State Growth, Parks and Wildlife Service, NRM South, DEP, Crown Land Services, and the Derwent Valley Council. In 2021 additional funding from the Weed Action Fund’s Large Grant was provided to the Derwent Catchment Project to help tackle the core infestation at New Norfolk that is proving particularly dense and difficult to access.

Did you know?

For the first time, seagrass restoration is being trialled in the Derwent at Windermere Bay thanks to a University of Tasmania PhD student with support from OzFish.

Recent management actions

The Derwent Estuary Program provides science to guide management. Here are several management examples highlighting how pollution has and is being, reduced in the Derwent Estuary and catchment.

Saltmarsh signs – wasteland to wonderland

Old Beach Foreshore Trail signage. Image: Derwent Estuary Program.
Old Beach Foreshore Trail signage.

Image: Derwent Estuary Program

Brighton Council in partnership with the Derwent Estuary Program and UTAS installed a suite of informative signs along the Old Beach Foreshore Trail as part of a community event in the summer of 2022.

The saltmarshes, and Derwent River foreshore are an extremely valuable natural asset within the Brighton Municipality and provide a great place for connecting with nature to learn about the many ways they benefit our wellbeing.

Saltmarsh is not only beautiful, it plays an important role in filtering water to keep it clean, it’s a place for fish and birds to breed and we now know it is very efficient at capturing carbon. Recently we surveyed 15 saltmarsh sites across the estuary to assess condition and recommend management options, including at Old Beach.

We are delighted to support Brighton Council in their efforts to look after this excellent area of saltmarsh. Once viewed as muddy wastelands, the new signs highlight how valuable saltmarsh is so we can instead celebrate this unique habitat and landscape.

We are lucky here in the Derwent to still have these remnant patches of nature in the midst of suburbia, such as the Old Beach Saltmarsh. We will do well to look after this special place and let it look after us in turn.

Boat wake monitoring methods

Boat wake on the River Derwent. Image: Peter Mathew / TasWater.

Image: Peter Mathew / TasWater

Boat wakes can have a variety of impacts, such as shoreline erosion and sediment resuspension that may affect plants and animals in the estuary and infrastructure located on the shore.

The 2021 commencement of a ferry service between Hobart and Kangaroo Bay (Bellerive), provided an ideal opportunity for the Derwent Estuary Program (DEP) to initiate a program to assess monitoring methods of susceptible local shores from ferry wake.

A project was proposed to investigate the usefulness of quick, efficient and low-cost methods, such as photo point monitoring, to track shoreline changes over time with funding support from the Department of State Growth.

12 representative sites on the Kangaroo Bay shoreline were chosen, and photo point monitoring of wake effects from the new ferry were conducted for three months. The project started prior to commencement of the ferry service to ensure baseline data for comparison. No monitoring was required on the Hobart Port side, as all the foreshore in that area of the river is hardened.

The outcome of the three months of surveying, suggested that the Kangaroo Bay shoreline has probably been impacted by the ferry at one of the study sites, while most of the changes detected at other sites were likely due to natural wind-wave events.

The lessons learnt, and observations made, during this project will be useful when it comes to any expansion of the Derwent estuary ferry network. It will assist in identifying shoreline types and substrates most prone to erosion owing to wave attack monitoring prior to launching additional ferries can be done. Management options for reducing foreshore impacts include adjusting boat speed to reduce wave intensity.

Clean Up Australia Day

Clean Up Australia Day volunteers at Prince of Wales Bay, Glenorchy. Image: Derwent Estuary Program.
Clean Up Australia Day volunteers at Prince of Wales Bay, Glenorchy.

Image: Derwent Estuary Program

The Derwent Estuary Program organised three Clean Up Australia Events during the national clean up week — Prince of Wales Bay in Glenorchy, and Cleburne Point and Shag Bay in Clarence.

With over 70 volunteers working for a total of 5 hours we collected a huge amount of rubbish from the beaches of the estuary. Twenty four full garbage bags plus a truck load of other larger items.

Much of what we pulled out was unrecognizable from being in the environment for a long time. Out of the mud, sand, washed up seaweed, rocks and saltmarsh we took away approximately:

  • 7500 pieces of soft plastic bags and wrappers
  • 600 plastic bottles
  • 50 surgical masks
  • 600 cans
  • 70 plastic straws
  • A lot of polystyrene — from big packaging slabs to tiny separate balls
  • Many other items such as fishing line, packing straps, a safe! Building materials, tyres, broken glass, plastic take-away food containers and cups, and cigarette butts.

In Prince of Wales Bay, which is surrounded by industrial uses and a marina, the coastal habitat is very compromised. The workers, many from the businesses nearby and the marina, got right into the mud and grime to remove a huge pile of rubbish — we didn’t really even get started on the small plastics all over the coast.

At Cleburne Point the rocky beach and human made wall that forms the point and provides habitat for birds, crabs and other estuary creatures, captures rubbish coming down the river and from the illegal dumping site here. The group of 16 volunteers, including Conservation Volunteers Tasmania, Sea Shepherd, and state government staff, collected most of the big items and the beach looked quite good after our visit.

At Shag Bay, another group of 16 volunteers from the community had the pleasure of paddling around from the jetty at Geilston Bay with Roaring 40s Kayaking company’s Luuk and Oliver who volunteered their time and gear. It was a very hot day so great to be on the water. This little bay is a lovely piece of natural habitat and the group worked mostly picking bits of broken down plastic and other rubbish from in the top of the bay in the saltmarsh — very satisfying to be leaving a functioning ecosystem healthier. A tyre and piece of treated wood post even got strapped on kayak decks and paddled out!

The best way to help the River Derwent is to prevent litter reaching it as clean-up events are the last resort. We hope our efforts this week inspire others to reduce our use of single use plastic, as well as become involved in future clean-up events.

Tasmanian Stormwater Policy Guidance and Standards for Development

Tasmanian Stormwater Policy Guidance and Standards for Development document cover.

The DEP facilitated a working group with councils, Tamar Estuary and Esk Rivers Program and the Local Government Authority of Tasmania to create a policy guidance document to streamline stormwater management in Tasmania.

The policy guidance document will inform and assist Councils acting as a Planning Authority to regulate development through the Tasmanian Planning Scheme and under the Urban Drainage Act. The TSPG can be used by Councils and developers and it is presented in two parts. Part 1 outlines the ‘why’ of stormwater management and Part 2 outlines ‘how’ to define stormwater management requirements with legislation.

View the Tasmanian Stormwater Policy Guidance Document

Volunteers clean up the invasive Northern Pacific seastar

Northern Pacific seastar cleanup volunteers. Image: Benni Vincent.
Northern Pacific seastar cleanup volunteers.

Image: Benni Vincent

Benni Vincent is leading a group of volunteer divers to reduce the impact of the predatory Northern Pacific seastar in the Derwent Estuary. The volunteers are concentrating on areas where the seastars are in really high densities such as under jetties and places inshore where there’s a lot of human activity. The volunteers are also focusing on areas where the endangered spotted handfish live because the seastars are known to eat the substrate they breed on.

The invasive seastar was introduced to Tasmania via ballast water from Japan and now the seastar is found right up the east coast of Tasmania and down to the D’Entrecasteaux Channel, but by far the worst area is the mid section of the Derwent Estuary.

The volunteers are involved for the long-term and understand that eradication is not possible, however if the numbers of the starfish in the hotspots around jetties and inshore areas can be reduced, next time their breeding success won’t be so high.

So far…

  • Clean ups conducted: 44
  • Approximate weight to date: 7009 kg
  • Estimated number of seastars: 159,378 (a lot!)

Keen to be involved?

This is a volunteer group and snorkelers, divers and non-divers are welcome. Join the Facebook group.

State of the Derwent 2020

State of the Derwent Estuary 2020 Update cover image

Hobart is a capital city that boasts swimmable beaches, an extensive network of walking and cycling tracks and great opportunities for boating and fishing. A major report looking at the health of the Derwent over the past five years has found the condition of the estuary has improved in some areas and declined in others.

This report highlights areas we can work on to maintain the health of this highly valued waterway. The State of the Derwent report summarised trends in industrial, sewage and stormwater discharges, monitoring results for swimming beaches, heavy metal levels in sediments and seafood, and the condition of key habitats and species. The report also highlights actions taken to clean-up the Derwent during this time.

Find it at State of the Derwent 2020.


Image: iStock / narcisa