The deaths were likely caused by low oxygen levels as flooding eases, a situation exacerbated by fish needing more oxygen because of the warmer weather, the department said.

Residents of the Outback town of Menindee complained of a terrible stench from the dead fish.

“We just started cleaning up, and then this happened, and it’s like walking around a dried-up mess and then you smell this putrid smell. It’s a horrible smell and horrible to see all those dead fish,” says Jan Dening, a resident.

Wildlife photographer Geoff Looney found huge clusters of dead fish near the main weir in Menindee on Thursday night.

“The stench was terrible. I almost had to put on a mask,” said Looney. “I was worried about my own health. That water comes all the way up to our pumping station for the city. People north of Menindee say cod and bass are floating in the river everywhere.”

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

Massive fish kills occurred on the river near Menindee in late 2018 and early 2019 during severe drought, with locals estimated to have killed millions.