Floods cause fish death and odours in Northern NSW


Floodplains in much of coastal of NSW have been inundated with floodwaters after the recent heavy rain. With the current warm, sunny weather, this could lead to ‘blackwater’ events, with some fish deaths and odours already reported.

EPA Director Regulatory Operations Adam Gilligan said odours from the receding floodwaters are likely to be caused by natural processes due to low levels of oxygen in the waterways.

“The recent significant rain elevated water levels in many rivers. As the floodwaters moved over low-lying areas surrounding the rivers, they picked up large quantities of organic matter, including decaying vegetation and leaves, as well as dirt, sand and other debris,” Mr Gilligan said.

“Although an important process for healthy river function, the decomposition of organic matter depletes oxygen levels in the water and releases tannins which give the water a distinctive black colour.

“This natural process is commonly known as ‘blackwater’ and is likely to cause fish deaths in rivers. The stagnant water also contributes to the odours which can smell like rotten egg gas.

“Blackwater after flooding is a natural feature of Australian river systems and the capacity to prevent and manage the impacts of blackwater is limited.”

The EPA was out sampling on Tuesday 30 March 2021 in the Hunter River from Raymond Terrace to Sandgate and has identified low oxygen levels. There are already some dead fish and odours present.

Mr Gilligan said the EPA is supporting local councils and other government agencies by sampling flood waters and providing advice.

“The EPA will continue taking samples and testing water sites where required. We encourage the community to be patient and to make contact with the EPA if they have concerns about polluted waterways in their area,”

River water may contain infectious organisms and chemicals, particularly after heavy rain periods. Entering river water after heavy rain increases the risk of injury and infection. People choosing to swim in rivers at any time should avoid swallowing water. If you have swallowed river water and become ill, seek medical advice.

If you are concerned about polluted floodwaters please contact Environment Line on 131 555 or by email at

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