The precise number of fish killed by the event was hard to determine since many of the deaths involved bony bream - small herrings that local birds were busy picking off. Murray cod, which can grow a metre or longer, are also among species found dead along the riverbank.

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"It's a bit of a feast - part of the boom and bust of the Darling," Mr Ellis said.

The immediate cause of the fish kill - the fourth in the state in recent months - was the sudden death of much of the blue-green algae that had been building up in near-stagnant waters.

A cool change dropped temperatures 15 degrees over the weekend after a long hot spell. As the algae died, they reduced the already low oxygen in the water to below critical levels.

"It added more stress to already stressed fish," Mr Ellis said.

The Bureau of Meteorology predicts the mercury will climb back into the 40s at Menindee by Friday at the start of another heatwave of at least four days' duration.

'Serious ramifications'

While the government and the Murray Darling Basin Authority largely blame the drought across NSW, environmental groups and local graziers say Menindee Lakes were unnecessarily drained twice in two years, removing fresh water supplies that could have lowered the risk of widescale fish losses.

"This is going to have serious ramifications," said Rob Gregory, a Menindee-based tour operator whose images and video have helped publicise the large fish kill.

"This [area] is the womb of the yellow belly" for the whole basin, he said, referring to the threatened fish species - also known as golden perch - that is also among the fish found dead near Menindee.

Researchers have extracted otoliths - or ear bones - from dozens of fish in a bid to understand more about the fish, and better target future releases of environmental water.

The bones can reveal how old the dead fish were, and even which part of the river system they came from, Mr Ellis said.