A major effort is underway cleaning in Australia after him death of millions of fish in a river in the west New South Wales.

The fish will be cleared from “areas of high density,” but it won’t be possible to remove them all, police say.

Police assured local residents that the water supply remained of “high quality”.

The deaths are believed to be caused by low oxygen levels in the river following a recent heat wave.

An emergency hub has been set up in the west Wales town of Menindee to coordinate the response and monitor water quality.

Describing the operation as “very challenging and important”, the area’s police chief, Brett Greentree, said the event was “unprecedented in terms of the millions of fish that have died”.

“Our goal is to make sure Menindee has clean water … We’re in a good place in terms of water quality right now,” he told reporters.

Commander Greentree said contractors with specialized skills would use “a net process” to remove the fish.

Posting a video on Facebook showing rotting fish lining the banks of the river, resident Graeme McCrabb wrote “the worst is yet to come”.