A major fresh fish kill has occurred at a weir in far western New South Wales.

Locals believe about one million fish may have died at Menindee Weir pool near Broken Hill.

Video shows dead fish floating on top of a vast area of water.

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Fish kills are defined by the NSW Department of Primary Industries as a sudden mass mortality of wild fish.

"Fish kills can occur at any time although data indicates fish kills are more likely to occur in summer or following sudden changes in temperature," the department said on its website.

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It's not yet known what has caused the fish kill but expert Joy Becker, an associate head of research at the School of Life and Environmental Sciences at the University of Sydney, said there were several possible factors.

"We need to let the fish kill investigation get underway to determine the cause," she said.

"This will help prevent and manage future scenarios that have a high likelihood of a fish kill occurring.

"Ultimately, fish kill events happen because the quality of the environment cannot sustain fish life.

"Causes of fish kills can be environmental, chemical, or possibly related to infectious disease agents including opportunistic pathogens or a combination of all these factors.

"When the water quality drops as is the case in blackwater events, it compromises fish physiology and if the drop is too sudden, too severe and/or too prolonged, this will lead to death.

"Complicating factors in blackwater events like disease can exacerbate situations of poor environmental conditions to cause major fish kills.

"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."