Hundreds of thousands of dead fish have been discovered in the Darling River at Menindee, in NSW's far west, in the latest mass fish kill to hit the region.
Bony bream make up the majority of the fish species found so far, however, locals have also spotted a large amount of cod and golden perch among the dead.
In 2019, during NSW's devastating drought, millions of dead fish were found in the area, putting the management of the Murray-Darling Basin under the microscope.
NSW's Department of Primary Industries (DPI) fisheries arm is dispatching officers to the area to assess the problem.
It says the bony bream species normally experience a "boom-bust" population cycle, especially when there is immense flooding.
But never on the scale seen right now.
Cameron Lay, director of freshwater environments at DPI Fisheries, said it was a "very distressing" situation.
He said the number of dead fish found so far suggested the mass kill could be at least "on par" with the 2019 event.
"The reports from late yesterday, early this morning … [suggested] we were looking at thousands, potentially tens of thousands, of predominantly bony bream — which is a native species — that have died," he said.
"Those estimates are well and truly into the millions now. We are seeing tens of kilometres where there is fish really as far as the eye can see, so it's quite a confronting scene."
Bony bream populations flourish quickly and die off with changing conditions but the level of deaths found today "probably exceeds things that we've seen before in that pattern", he said.
"In and among those bony bream we are starting to see an increasing number of golden perch and even a few Murray cod," Mr Lay said.
"These are longer-lived large-bodied native species.
"This is more of a concern that bony bream are more of a short-lived species and they rely on booming and good times and then having these sort of rapid die-offs."
He is concerned about a heatwave expected to roll through western NSW in the coming days, with temperatures to exceed 40 degrees Celcius.
"That in itself can present an ongoing risk to water quality and native fish so we will be doing everything we can to monitor the situation and use whatever management options we do at our disposal," he said.
Local Graeme McCrabb said the water quality in the area was "appalling" recently.
"All I can see is carp sucking for air," he said.
"I think we would well and truly be into the millions of dead bony bream again."
Mr McCrabb questioned the management of the water system ahead of a possible El Niño phase which could bring drought back to the area.
"I don't see how the river can be fit for purpose for showering and cleaning when you've got dead fish and the smell of dead fish, it's just putrid."
A WaterNSW spokesperson said it had increased monitoring the Menindee Lakes area in recent months to track the impact of prolonged flooding in the region.
They said flooding typically results in reduced oxygen levels as floodplain water flows back into the river channel.
"WaterNSW is working to support agencies such as DPI Fisheries by operating the lakes to ensure the best quality water is released downstream."
Menindee, a town of about 500 people, is located an hour's drive from Broken Hill.
It has been the site of several mass fish kill events in recent years, with a 40-kilometre algal bloom blamed for the death of one million fish in 2019.
The 2019 event coincided with a period of high temperatures in the state's far south, and drought along the river system.
Last year, thousands of fish were again found dead at Menindee lakes.
Claims of water mismanagement and poor river health have been raised by locals as a potential cause of the mass deaths, but the DPI has denied this in the past.
Residents criticised a decision by the Murray Darling Basin Authority to drain the Menindee Lakes in 2014 and 2017, to meet water demands downstream.
Joy Becker, an associate professor with the University of Sydney, said fish kill events could occur due to a sudden, severe or prolonged drop in water quality.
"Ultimately, fish kill events happen because the quality of the environment cannot sustain fish life," she said.
"Causes of fish kills can be environmental, chemical, or possibly related to infectious disease agents including opportunistic pathogens or a combination of all these factors."
She said investigations into the cause of this incident could help identify how to prevent future kill events.
