"Reduced opportunities for breeding are likely to result in significant reductions in ibis productivity in the long term," the paper said. Water resource development had already cut the juvenile ibis numbers by an estimated 2.3 million.

"It used to flood fairly frequently [in the Narran Lakes]," said Kate Brandis, a research fellow at the Centre of Ecosystem Science at the University of NSW and lead author of the paper.

"That doesn't tend to happen any more."

Narran Lakes is one of 16 basin wetlands listed for their international importance under the Ramsar Convention, a treaty to which Australia is a signatory. The region is home to 46 waterbird species and, in 1983, supported the largest ibis colony every recorded in Australia, about 400,000 pairs, the paper noted.

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The researchers studied the lakes' straw-necked ibis - distinct from the white ibis, common in cities such as Sydney - because they were "by far the most dominant" waterbird and Narran had the largest concentration of them, Richard Kingsford, a UNSW professor and co-author of the paper, said.

Waterbirds such as ibises "are really good indicators of river flows and ecosystem health, and their breeding essentially is a barometer of how well that wetland is doing", Professor Kingsford said.

Floods need to be big enough - greater than 154 billion litres - and last long enough to attract waterbirds and provide food for them to forage, court mates, build nests, and rear and fledge chicks.

Ibises need three to five months of inundation for successful breeding cycles.

However, with the expansion of irrigation, particularly upstream around St George in Queensland since the 1990s, major floods reaching the Narran Lakes are been reduced.

"The water resources have sort of taken off the really big floods, so we end up with more frequent smaller-sized floods of smaller volume and smaller duration," Dr Brandis said.

"The big ones are nowhere near as large in terms of volume or in duration."

Maintaining ibis numbers has implications for farmers. The birds are known as a "farmer's friend", feasting on the locusts and grasshoppers that would otherwise consume crops.

Despite those benefits and Ramsar obligations, the federal government recommended a cut of 42 billion litres of year to the rivers supplying the lakes.

That reduction was formalised this week with passage of the northern basin review amendments cutting environmental water savings by 70 billion litres.

A spokeswoman for the Murray-Darling Basin Authority defended the plan.

"Ecological modelling work by the [authority] clearly demonstrates that the Narran Lakes will experience around 30 per cent more bird breeding events due to the Basin Plan," she said.

Environmental flows provided through the plan "will ensure the Narran Lakes vegetation is in better health and is better able to support migratory birds when they arrive at the lakes."

But Dr Brandis said the latest reduction"will just exacerbate what we've seen".

The researchers' modelling did not take into account future climate change - nor does the basin plan. The plan was also based on "best guesses" about wildlife, she said.

"There’s a huge lack in our understanding of the basic biology of some of these birds," Dr Brandis said. "We don’t know how long they live, how frequently they need to breed ... to account from them accurately in any modelling of water needs."