An emergency response centre has been put into action in the state's far west to coordinate the clean-up of the worst mass kill ever to hit the region.

Millions of decomposing fish have been flowing down the Darling River in Menindee in recent days, with fears it could cause an ecological disaster that will take years for the river to recover from.

The Menindee-based Emergency Operations Centre (EOC), overseen by NSW Police, will work with other agencies to remove and dispose of the fish, and maintain clean water flows to the town and surrounding areas.

"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," State Emergency Operations Controller, Deputy Commissioner Emergency Management Peter Thurtell, said in statement.

"Importantly, there is ongoing testing of the water quality, which will ensure immediate action if a switchover for supply is required."

The incident, which surpasses similar events in 2018 and 2019, was caused by low oxygen levels in the water — known as hypoxic blackwater — and made worse by high temperatures which reached close to 40 degrees Celsius on Sunday.

In Sunday's statement police said state agencies were "continuing targeted releases of higher quality water where possible to boost dissolved oxygen levels in this area".

The NSW Department of Planning and Environment, Central Darling Shire Council and WaterNSW are among the agencies involved.

"Numerous agencies are already on the ground in the area, with more on the way to conduct and support operational activities," operations lead NSW Police Force Regional Emergency Operations Controller, Assistant Commissioner Brett Greentree, said.

He said that significant planning, including risk assessments were already underway.

"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."

Locals concerned with the overpowering smells and water quality had been calling for authorities to act.

Stephen Howse uses the river to irrigate his vineyard and is worried about the impact on his crops and equipment.

"There'd have to be a million fish dead along a kilometre of the river. It's not pleasant," he said.

"There's plenty of people here who don't have the bore water, they rely on this water for everything in the house — washing, showering, how would you like to shower in that water at the moment?"

A community meeting is expected to take place on Tuesday, where the community will be updated on the latest developments, and experts will be on hand to answer questions.