NSW Police have established an emergency operations centre at Menindee to co-ordinate a multi-agency effort to clean up millions of dead fish.
The fish deaths have been attributed to hypoxic blackwater, a naturally occurring phenomenon that causes extremely low dissolved oxygen levels, police said in a statement on Sunday.
State Emergency Operations Controller, Deputy Commissioner Emergency Management Peter Thurtell, said the immediate focus was to provide a clean water supply to residents.
"There is no need for community concern as the initial assessment has determined multiple viable solutions to maintain water supply to the Menindee township and surrounds," he said.
"Importantly, there is ongoing testing of the water quality which will ensure immediate action if a switchover for supply is required."
Mr Thurtell said the second tier of the operation was to remove the dead fish, numbered in the millions.
Residents will be briefed on the operation at a community town hall event on Tuesday.
NSW Police Force Regional Emergency Operations Controller, Assistant Commissioner Brett Greentree, said the clean up of dead fish around people's homes would start this week.
"Significant planning, including risk assessments, is already under way for clean-up efforts, which will include the removal of as many of the dead fish as possible, prioritising the immediate areas around Menindee, as well as other areas deemed high-risk in terms of water supply," he said
"While this will require specialised skills and equipment, experts in the EOC have already identified suitable contractors and it is expected work will commence this week."
NSW government agencies are continuing targeted releases of higher-quality water where possible to boost dissolved oxygen levels in the area.
The NSW Department of Planning and Environment said in a statement the die-off of the fish, mostly bony herring, was caused by extremely low dissolved oxygen levels recorded in the river earlier this week after recent floods and hot weather.
Politicians including federal opposition spokeswoman for water Perin Davey have called for the urgent removal of the dead fish.
Senator Davey suggested aspects of the National Carp Control Program, created after six years of research into carp biocontrol strategies, could be adapted for the clean-up.
"We know one of the biggest concerns for carp biocontrol was what to do with the dead fish," she said.
"The current natural event that we are seeing, with hundreds of thousands of dead fish floating along the Darling, provides an opportunity to test the clean-up options that scientists have considered under the carp control program.
"The current event is too urgent to go back to the drawing board. We need urgent action," she said.
The NSW Greens' water spokeswoman Cate Faehrmann said the fish needed to be removed before they decomposed and caused an ecological disaster.
"This is categorically a catastrophe, regardless of whether this is a consequence of receding floods or water mismanagement," she said.
"The NSW and federal governments should be acting now to clean up the millions of rotting fish which are spanning kilometres of the river."
A 40km algal bloom in the same part of the river was blamed for the death of a million fish in 2019.
The 2019 event coincided with a period of high temperatures in the state's far south and drought along the river system.
This latest fish die-off is expected to exceed the 2019 event.
