Fish kills and odours

Recent heavy rainfall and severe weather across NSW can lead to ‘blackwater’ events, with fish kills and odours likely to follow.

Odours from the receding floodwaters are likely to be caused by natural processes due to low levels of oxygen in the waterways.

The significant rainfall seen over parts of the State has elevated water levels in many rivers. As the floodwaters move over low-lying areas, they can pick up large quantities of organic matter, including decaying vegetation and leaves, as well as dirt, sand and other debris.

Although this is a natural process for healthy rivers, 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 as these blackwaters drain back to the rivers and flow downstream, is likely to cause fish kills in the rivers. The stagnant water also contributes to the odours which can smell like rotten egg gas. Unfortunately, the capacity to prevent and manage the impacts of blackwater is limited.

We are supporting local councils and other government agencies in flood-impacted areas and will continue to sample and test waterways impacted by floods and provide advice.

River water may contain infectious organisms and chemicals, particularly after heavy rain periods.  Entering river water after heavy rain periods increases the risk of injury and infection.  If you choose to swim in rivers you should avoid swallowing water. If you have swallowed river water and become ill, you should seek medical advice. Find out about water safety around flood waters.

We encourage the community to be patient and to contact Environment Line on 131 555 or at with any concerns about polluted waterways.