Environmentalists are concerned the amount of water diverted to irrigators from Australia's third longest river could lead to more mass fish deaths in far western NSW.
Up to one million fish have died in the Darling River system at Menindee, sparking an investigation by the NSW Department of Primary Industries and WaterNSW.
Golden perch, Murray cod and bony herring are among the species affected.
The DPI said ongoing dry conditions and a recent big temperature drop could have caused the large fish kill.
But University of NSW environmental science Professor Richard Kingsford believes while the drought has contributed the problem has been a long time coming.
He argues it's due to the diversion of water for irrigation and extractions from the Menindee Lakes.
"These types of events are likely to become more frequent, given there is a lot of diversion of water for irrigation upstream," Prof Kingsford told AAP on Tuesday.
"The drought would not have as much impact if there was more water in the river and if there was more water in the river, the water quality would be better and the fish probably wouldn't be as badly affected."
Environmental group Inland Rivers Network says mismanagement impacted the health of the river before the drought.
"(It is) an extreme drought but because any flows that might have got into the Darling River are being used somewhere else, it has exacerbated the problem," network president Bev Smiles said on Tuesday.
NSW Primary Industries Minister Niall Blair says it's a "cheap shot" to criticise irrigators because no one is pumping water unless they're allowed.
He argues the Murray-Darling Basin Authority authorised a water release from the Menindee Lakes to South Australia and then the drought hit which meant there was no water to replenish the system.
Once the lakes contain more than 480 gigalitres of water the state government doesn't control it as the flow is shared with other states.
"If no one let the water out there would be more water there but it was asked to be let out," Mr Blair told AAP on Tuesday.
"We want to reconfigure those lakes through engineering solutions and also talk to Canberra about the rules so we don't keep pulling the plug and then not turning the tap on, so we can have better outcomes."
Ex-Greens and now independent MP Jeremy Buckingham claims the NSW government and the MDBA's "rapid" draining of the lakes allowed upstream corporate irrigators to avoid contributing water to the environment.
He warns the Darling River and the fish that live in it will all die if there isn't an overhaul of the basin's management.
The latest kill follows an incident in December resulting in more than 10,000 fish mortalities along a 40 kilometre stretch of the Darling River.
A royal commission was held in 2018 to investigate allegations of widespread water theft from the Murray-Darling Basin by rogue irrigators with its report expected on February 1.
Australian Associated Press