More than three tonnes of dead fish have been scooped out of the Darling River in NSW's far west in recent days as locals, irrigators and politicians argue over who's to blame for the catastrophe.

The NSW Department of Primary Industries estimates close to one million fish died over the weekend - the latest in a series of mass fish deaths in the area this summer.

Officials are also investigating the death of thousands of fish on the Murrumbidgee River in Redbank Weir about 100km west of Hay.

The DPI warned hot and dry conditions pose ongoing threats to native fish over the remainder of summer.

Freshwater ecologist and Murray-Darling basin expert Angus Webb says the Menindee fish kills are unprecedented in terms of deaths in one localised area.

But he says they might not be unprecedented for the basin because a similar amount of fish likely died in dozens of mass death events during the 2006 and 2007 drought.

The researcher of large-scale environmental problems in freshwater systems said the Menindee fish deaths were "deeply saddening" but urged policy-makers to avoid making rash decisions.

"A longer-term approach to this - learning from these events, looking how we use the water for environmental and agricultural purposes - should bear fruit," the University of Melbourne associate professor told AAP on Wednesday.

"Now that's really difficult to say to someone living in the Lower Darling surrounded by dead fish, but it is the way we'll improve gradually."

NSW Regional Water Minister Niall Blair, who visited Menindee on Tuesday to see what he described as an "environmental catastrophe", rejected suggestions the state could have stopped water being released from the Menindee Lakes.

Deputy Prime Minister and Nationals Leader Michael McCormack defended the commonwealth's role in managing the river system arguing the Murray-Darling Basin Authority stopped taking water when the lakes fell below 480 gigalitres.

After that threshold is reached, responsibility reverts to the NSW government.

"Governments don't make it rain ... the fact is it is very, very dry there," Mr McCormack told the Nine Network.

Assoc Prof Webb said people didn't understand how much was asked of Australia's largest river system which is used for agricultural irrigation and therefore less robust to adverse events such as drought.

He said the "world-leading" basin plan was working as intended, but it had no manual and required constant monitoring, research and adaptation.

"It's an imperfect solution to an unsolvable problem," Assoc Prof Webb said.

"We have to be used to the idea that poor ecological events are a feature of the choices we've made about what we want from this system."

The academic said balancing environmental and irrigation flows were difficult given "we get a hell of a lot from irrigated agriculture".

Australian Associated Press