Members of the public are being urged to help track the extent of a fish kill in a region hit hard by ex-tropical cyclone Alfred.
Thousands of fish have been washing up dead or gasping on the banks of the Richmond River at Ballina in northern NSW, according to habitat charity OzFish.
Locals have been warned to brace for more fish kills as the effects of the ex-tropical cyclone continue to wreak havoc a week after it crossed the coast, triggering widespread flooding in northern NSW and southeast Queensland.
A NSW government fisheries spokesperson said an investigation was under way into reports of the kill.
"Fisheries staff are liaising with local councils on monitoring water quality and the fish deaths, with several locations impacted," they said.
"Ongoing fish kills in this region can be expected over the coming days and into next week."
OzFish chief executive Cassie Price said black water containing very low levels of oxygen was likely the cause.
Contamination happens when organic matter is washed into waterways, turning a dark colour and stripping water of oxygen as it decomposes.
Photos supplied by the charity appear to show fish washed up on the riverbank, while it has also received reports of mud crabs and eels emerging from the water to escape the contamination.
Ms Price said the results of water tests carried out by volunteers "aren’t good".
"Healthy water should have at least 5mg of oxygen per litre," she said.
"Fish experience distress when it falls below 4mg per litre and start to die at 2mg a litre.
"In Richmond River today, the dissolved oxygen levels are almost nil, sitting at 0.4 mg a litre.”
Ms Price urged locals to record and report any dead fish they found to the charity so it could understand the scale of the deaths.
“Record a pin drop or geo reference of your location, estimate the number of fish, take note of any key species you can identify, and take photos and videos," she said.
Dozens of fish kills are reported in NSW each year but some events are more devastating than others, including the estimated 20 million fish dead in Darling River at Menindee flooding in March 2023.
Ms Price believes restoring swamplands around the Richmond would help prevent similar events in future.
"Urgent action must be taken to prevent further fish kills and build resilience into our waterways," she said.
