Contractors called in to dispose of up to a million dead fish in a far west NSW river could have a window of only five days before the carcasses trigger a second wave of deaths.

Key points: The fish were killed by a toxic bloom of blue-green algae

The fish were killed by a toxic bloom of blue-green algae A contractor will begin removing the carcasses early next week

A contractor will begin removing the carcasses early next week A council says there is a window of only five days before the fish sink to the bottom and can't be disposed of

The rotting carcasses are the result of a toxic algal bloom in a 40-kilometre stretch of the Darling River at Menindee, near Broken Hill, which robbed the water of oxygen.

The fish began to appear at the beginning of this week, but the clean-up process is still yet to begin after various authorities butted heads over who was responsible.

The local council and the Department of Primary Industries agreed to divvy up the workload — the department is engaging a contractor to remove the fish from the water, and the council will take the fish from the riverbank to landfill.

"We have a reasonably short window of opportunity to get rid of [the fish carcasses]," said Greg Hill, the general manager of the Central Darling Shire Council.

"I'd probably say up to five days to get them out of the water and dispose of them."

A contractor will remove the fish from the water and work is due to begin early next week.

Mr Hill says it's taken so long because there are very few contractors with the resources to deal with a problem of this scale.

Cleaners in a race against time

If the fish aren't cleaned up soon they'll sink to the bottom of the river or decompose in the water, making their remains impossible to remove.

"There's a number of them that have probably sunk already but there's still quite a few still floating around on the surface," Mr Hill said.

Sorry, this video has expired Viral video of mass fish kill

The consequences of not removing the fish could be severe.

Environmental and civil engineering professor Stuart Khan studies water pollution and said the bacteria that break the fish carcasses down will once again rob the water of oxygen.

"Tragically the most likely effect of those fish breaking down would be to lead to the same low oxygen concentration that killed the fish in the first place," he said.

"We could see a second wave of fish kills."

He also says the fish corpses contain nutrients such as nitrogen and phosphorus that could trigger another algal bloom.

Hundreds of thousands of native fish were found dead. ( Supplied: Rob Gregory )

Calls for a royal commission

The community of Menindee has been hit hard by the mass deaths.

Mr Hill says the news could deter anglers for potentially years to come, and the rotting fish in the river have disrupted the lives of locals.

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"The smell it makes, the river is only several hundred metres away from homes and in some cases there are homes right on the river, so the smell from decaying fish and the quality of water isn't pleasant," he said.

Greens Senator Sarah Hanson-Young said locals could no longer use water from the river for their households.

"This is a river system in crisis and it's dying before their eyes," she said.

Ms Hanson-Young called for a federal royal commission into the matter, blaming water-thirsty corporate irrigators upstream who hadn't "played by the rules".

Cotton Australia previously rebutted criticism by blaming the disaster on poor water management of the Barwon-Darling Basin.

Earlier this week independent state MP Jeremy Buckingham travelled to the site and filmed himself holding a rotting Murray cod, describing the situation as a "disgrace".

Professor Khan was also shocked.

"The scale of what's currently happened is, as I understand it, unprecedented," he said.

"It's a massive environmental disaster."

A spokeswoman for Primary Industries and Regional Water Minister Niall Blair said it probably won't be the last mass fish kill, either.

"Unfortunately another kill is likely as hot conditions are forecast over the coming days, as warm water results in reduced levels of dissolved oxygen available for fish," the spokeswoman said.