CANBERRA, AUSTRALIA -- Millions of fish have washed up dead in southeastern Australia in a die-off that authorities and scientists say is caused by depleted oxygen levels in the river after recent floods and hot weather.

Residents of the Outback town of Menindee in New South Wales state complained of a terrible smell from the dead fish.

"The stink was terrible. I nearly had to put a mask on," said local nature photographer Geoff Looney.

"I was worried about my own health. That water right in the top comes down to our pumping station for the town. People north of Menindee say there's cod and perch floating down the river everywhere," he said.

The Department of Primary Industries said the fish deaths were likely caused by low oxygen levels as floods recede, a situation made worse by fish needing more oxygen because of the warmer weather.

Police have established an emergency operations centre in Menindee to coordinate a massive cleanup this week.

State Emergency Operations Controller Peter Thurtell said the immediate focus was to provide a clean water supply to residents.

"There is no need for community concern as the initial assessment has determined multiple viable solutions to maintain water supply to the Menindee township and surrounds," he said.

State agencies also started to release higher-quality water where possible to boost dissolved oxygen levels in the area.

"We've just sort of started to clean up, and then this has happened, and that's sort of you're walking around in a dried-up mess and then you're smelling this putrid smell. It's a terrible smell and horrible to see all those dead fish," said Jan Dening, a Menindee resident.

Mass fish kills have been reported on the Darling-Baaka River in recent weeks. Tens of thousands of fish were found at the same spot in late February, while there have been several reports of dead fish downstream toward Pooncarie, near the borders of South Australia and Victoria states.

Enormous fish kills occurred on the river at Menindee during severe drought conditions in late 2018 and early 2019, with locals estimating millions of deaths.