Millions of fish have washed up dead in southeastern Australia in a die-off that authorities and scientists say is caused by flooding and hot weather.

The New South Wales state Department of Primary Industries said the fish kills coincided with a heat wave that stressed a system that has experienced extreme conditions from large-scale flooding.

The deaths were likely caused by low oxygen levels as the flooding subsided, a situation made worse by fish needing more oxygen due to the warmer weather, the department said.

Residents of the inland town of Menindee complained of the terrible smell of dead fish.

“We just started cleaning up, and then this happened, and it’s like you’re walking in a dried-up mess and then you smell this putrid odor. It’s a terrible smell and it’s horrible to see all those dead fish,” said Jan Dening, a local.

Nature photographer Geoff Looney found huge clumps of dead fish near the main dam in Menindee on Thursday night.

“The stench was terrible. I almost had to put on a mask,» Looney said. “I was worried about my own health. That water right at the top goes down to our pump station for the city. People north of Menindee say there are cod and perch floating down the river everywhere.»

Mass murders have been reported in the Darling-Baaka river in recent weeks. Tens of thousands of fish were found in the same location in late February, while there have been several reports of dead fish downriver towards Pooncarie, near the South Australian and Victorian state borders.

Huge fish kills occurred in the Menindee River during severely dry conditions in late 2018 and early 2019, with locals estimating millions of deaths.