SYDNEY -- Residents in a remote Australian town are calling for authorities to act promptly and begin a clean-up, after hundreds of thousands of dead fish were found floating in the nearby Darling River.

Low levels of oxygen in Australia's second-longest river are to blame for the mass fish death late last week near the town of Menindee in New South Wales state, around 1,000 km (620 miles) west of the state capital Sydney, environmental authorities said.

It follows fish deaths in the same area in 2018 and 2019 where up to a million fish died from poor water flow, poor water quality, and sudden temperature changes.

"Other than rainwater ... we 100 percent rely on that river for our household domestic use," Karen Page, a Menindee resident, told state broadcaster ABC.

"As soon as they've seen what was happening, they should've had that equipment here. They should've already been cleaning this out."

New South Wales Police said late on Sunday that an emergency operations centre was being set up in Menindee to coordinate the disposal of the decomposing fish and to organise the supply of clean water.

Joy Becker, a professor of aquatic animal health at the University of Sydney, said it would take a significant amount of time for the river's ecosystem to recover.

"It does mean that those populations (of fish) may not rebound as quickly or at the same magnitude," she said.

"Pest species can actually just take over that spot, which makes it even harder for native fish to recover."