Lindy Hunt, a representative from Broken Hill-based Rescue and Rehabilitation of Australian Native Animals, said a member of her group also visited and found "everything's sorted".

"How can your heart not go out to kangaroo mired in mud like that?" she said. "They are pretty much doomed, these animals."

Even if extracted, the animals - particularly kangaroos - are exposed to so-called capture myopathy, where stress reaches such levels muscle fibre begins to dissolve, Ms Hunt said.

Mark Pearson, the NSW Animal Justice Party MP, said he visited the lakes in November and found emus "strewn around...right up beside the lakes dead".

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"This is really a disaster," he said. "We have an animal welfare crisis."

Mr Pearson said that while the drought was clearly an issue across the state, the Berejiklian government was the "main factor" in the Menindee region's problems - including the huge fishkill over the past week - because it had allowed the lakes to be drained twice in four years.

"I think it's a duty of care," he said, adding that authorities should send regular sorties to put down stressed animals now they know the worst regions.

Niall Blair, the primary industries minister, said "hindsight's an amazing thing", with governments and agencies such as the Murray-Darling Basin Authority acting on the best advice they had concerning Menindee-Lake releases.

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While it was distressing to see animals die, boom and bust periods were common in the Darling and other regions. Graziers, too, would have every incentive for animal husbandry, welfare and economic reasons to ensure their animals didn't stray into the bogs, whether at Menindee or elsewhere, he said.

With severe heat due to bake inland NSW during the coming few days, it was also possible further large fish kills will happen in regions not affected by the Menindee Lake draining, Mr Blair said.

Nick Moir said the euthanising of the animals he had photographed "is almost the best that you can expect".

He said readers had asked why he didn't go in but said "it's a dangerous thing to do", given how soft the ground is in that area.

"By and large there was a lot of support for bringing attention to the plight of the basin," Mr Moir said, adding that he had seen 50-60 other animals nearby, including emus, that might suffer similar fates to the roos and sheep.

The OEH spokesman said it was not practical to attempt to fence animals out of receding water bodies.

"People who come across animals suffering as a result of drought should advise the landholder responsible in the first instance," he said.

"This is the unfortunate reality of drought and the situation is not likely to improve until there is drought-breaking rain."