Sydney - The removal of millions of rotting fish is finally underway Wednesday in south-eastern Australia, almost a week after an unprecedented mass die-off.

Residents of Menindee, 1000 kilometers west of Sydney, say their town is facing a "funeral procession" of the millions of fish carcasses. A senior police officer in charge of the clean-up has said up to 20 million fish may have died.

Parts of the Darling River in the far west of New South Wales, Australia's most populous state, have been clogged with dead fish since last weekend.

Scientists believe low oxygen levels in the river following a recent heatwave caused the massive die-off. Experts say the native fish suffocated in the hypoxic blackwater. It is a naturally occurring phenomenon that causes extremely low oxygen levels.

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Australian police say removing all the dead fish will be a 'logistical nightmare."

Rotting carcasses stretch along several kilometers of the waterway, and the clean-up is underway. Contractors will use two types of booms to sweep the river to collect both fish on the surface and those already sinking.

The emergency response is being coordinated by New South Wales Police Commander Brett Greentree. Greentree told the Australian Broadcasting Corp. Wednesday that "The poor people of Menindee - who have not only had to put up with the floods recently, they have had a really tough time - and now all these decomposing fish, you know, it has been really tough for that community."

"I really feel for them," Greentree added.

The town suffered another mass fish kill in 2018, which was caused by extreme drought.

This time, it is the widespread flooding and excessive heat that are to blame. Authorities will investigate the deaths and will try to find the underlying causes.

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Scientists say the impact of climate change is making extreme droughts, floods and heatwaves more frequent in Australia.

The Darling River is part of the Murray-Darling Basin, which is Australia's largest river system.