Millions of dead fish have been found dead in an Australian river due to dangerously low oxygen levels in the water caused by receding flood waters, government officials said.

The fish were found in the Darling River, near the town of Menindee, and are mainly bony herring, though other species, such as murray cod, golden perch, silver perch and carp also died in an "ongoing" event, the agency said.

"The Bony Herring species typically booms and busts over time," the Department of Primary Industries in New South Wales stated in a social media posting. "It 'booms' in population numbers during flood times and can then experience significant mortalities or 'busts' when flows return to more normal levels

A heat wave in the area has further exacerbated the lower oxygen levels.

Mass fish kills were observed in late February on the Darling-Baaka River, The Associated Press reported. Additionally, mass fish kills occurred on the river at Menindee during severe drought conditions in late 2018 and early 2019.