Millions of fish have died in another mass kill in the lower Darling-Baaka river near Menindee, in New South West's far west.

Photos supplied by Menindee residents show dead fish - mostly bony bream, but also Murray cod, golden perch, silver perch and carp - blanketed across the river's surface.

It is the latest in a series of large-scale fish deaths that have prompted questions about the management of water levels in the Murray-Darling Basin. Menindee residents who spoke with Guardian Australia on Friday said the latest fish kill appeared larger than previous mass deaths.

About a million fish died during a sustained drought in the same area in 2019 after a rapid drop in temperature led to an algal bloom de-oxygenating the river.

Resident Graeme McCrabb said the scale of the mass kill on Friday was "unfathomable".

"It's horrendous here today," he said, speaking from the riverbank about 5km upstream of Menindee. "The river is just white. I'm looking at probably a kilometre or a kilometre-and-a-half of fish and they're all dead. It's unfathomable."

The NSW Department of Primary Industries said on Friday there was a "developing large-scale fish death event" affecting millions of fish below the Menindee main weir through to weir 32, adjacent to the Menindee township.

A department spokesperson said the deaths were due to low oxygen levels in the water as flood waters receded.

"Significant volumes of fish including carp and bony herring, nutrients and organic matter from the floodplain are being concentrated back into the river channel. The current hot weather in the region is also exacerbating hypoxia, as warmer water holds less oxygen than cold water and fish have higher oxygen needs at warmer temperatures," the spokesperson said.

"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."

The spokesperson said the bony bream population "typically booms and busts over time". "It booms in population numbers during flood times and can then experience significant mortalities, or busts, when flows return to more normal levels."

Geoff Looney, a photographer from Menindee, said the latest mass kill was much more severe than the fish deaths earlier this year. "This time there's hardly a live fish out there," he said.

McCrabb said the fish would have washed through to Menindee by Saturday. "Then it will be just dead rotting fish through the township and people won't be able to use the water," he said.

Menindee is a town of about 500 people around 100km south-east of Broken Hill. Locals have previously accused authorities of contributing to fish deaths by allowing the Menindee Lakes to be drained and not effectively managing water quality. Officials have denied that they were responsible.

Associate Prof Joy Becker, from the University of Sydney's School of Life and Environmental Sciences, said an investigation should determine the cause of the mass death.

"Ultimately, fish kill events happen because the quality of the environment cannot sustain fish life," she said. "It is important to remember that fish kill events impact not only the large-body fish like the Murray cod and bony bream, but also the small-body fish like the gudgeons that are essential to maintaining a healthy aquatic ecosystem."