A new scientific report leaked to the ABC warns parts of the lower Murray River may be beyond recovery without water by October.
But the Rudd Government has deferred consideration of the report until a meeting of the Murray-Darling Ministerial Council in November.
The report, prepared by a scientific panel and leaked to the ABC, warns there are six months to save crucial parts of the Murray-Darling Basin.
Without sufficient water, ecosystem recovery may take years to decades and the unique ecology of the lower Murray will be irreversibly lost, it says.
Vegetation on the lower Murray has been lost and wetlands are dry while some fish species might already be extinct.
Dr Arlene Buchan, healthy rivers campaigner for the Australian Conservation Foundation, says that waiting until November could be detrimental to the system.
"What the ministerial council has done is ignore the urgency that is portrayed by these scientists.
"They have more or less made a decision about the lower lakes and the Coorong by not making a decision to return water to them."
University of Adelaide ecologist, Associate Professor David Paton, says some fresh water lakes are on the verge of being unrecognisable.
"As far as I am concerned there has been 10 years at least that people have said you have got to restore the environmental flows to the system if you wish to keep the natural assets," he told ABC radio.