Millions of dead fish continue to flow down the Darling River in Menindee in NSW's far west as locals clean up the worst mass kill ever to hit the region. 

A constant stream of mostly bony bream has been floating down the river this morning, as well as golden perch and silver perch, and some Murray cod. 

Authorities have confirmed the overall scale and size of this mass fish kill eclipses similar events in 2018 and 2019.

The horrific incident has been caused by low oxygen levels in the water combined with high temperatures, expected to peak above 40 degrees Celsius in the area over the weekend.

The NSW Department of Primary Industries (DPI) said it would continue to assess the risks to fish health in the area through ongoing monitoring. 

"This event is ongoing as a heatwave across western NSW continues to put further stress on a system that has experienced extreme conditions from wide-scale flooding," it said in a statement. 

"The amount of dissolved oxygen water can hold decreases with increasing water temperature, which can add additional stress to fish that may already be struggling."

The low oxygen levels occurred after recent flood waters receded, dragging dirt and muck and bacteria back into the river, that is already over-populated with carp. 

Seven weeks ago, local resident Jan Dening remembers watching the water rising "very quickly". By 3:30am, her property was flooded.

"It was like a mini tidal wave coming down," she said.

She said the mass fish kill so soon after was hard to comprehend. 

"We've just started to clean up, and then this has happened. You're walking around in a dried-up mess and you're smelling this putrid smell."

Premier Dominic Perrottet said it was a very difficult situation and that his government was working "very closely" with the Menindee community. 

"Obviously off the back of floods it has caused significant challenges right across our regional communities," Mr Perrottet said. 

"We've seen the flooding hasn't just had impacts on our roads, it's had an impact on our river systems as well. 

"I think there is a lot of work that needs to be done. 

"It's a long-term challenge that lies ahead."

Menindee Local Aboriginal Land Council director Michelle Kelly said there was a feeling of hopelessness within the community, and frustration that no one had come to talk to them. 

She said the community were still in the dark following the previous fish kill events. 

"Nobody knows what's happened."

The DPI said the bony bream species normally experience a "boom-bust" population cycle, especially when there is immense flooding. 

But never on the scale seen right now. 

The Central Darling Shire Council said it had contacted Essential Water in Broken Hill and was monitoring the water supply. 

"Council has been advised that at this stage there is no risk to water quality," it said in a statement. 

During the state's devastating drought in 2019, millions of dead fish were found in the area, putting the management of the Murray-Darling Basin under the microscope. 

Opposition leader Chris Minns said it was devastating for a community that had suffered so much. 

"The river system in far western NSW has been mismanaged by over a decade of National Party leadership," he said. 

A WaterNSW spokesperson on Friday said it had increased monitoring the Menindee Lakes area in recent months to track the impact of prolonged flooding in the region.

Menindee, a town of about 500 people, is located an hour's drive from Broken Hill.

It has been the site of several mass fish kill events in recent years, with a 40-kilometre algal bloom blamed for the death of one 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.

Last year, thousands of fish were again found dead at Menindee lakes.