Millions of fish have died in an Australian river, as a result of ongoing high temperatures.

The relentless heatwave across Australia has caused flood waters to recede, leading to low oxygen levels in the water. The fish then died from hypoxia (insufficient oxygen).

Warmer water holds less oxygen than cold water, and fish also typically need more oxygen in warmer temperatures too. So the impact of both the ongoing heatwave, plus water levels receding has led to the fish mortality.

Experts say the main type of species that were found in the Darling River, near Menindee in New South Wales, are the Bony Herring, which do typically boom and bust depending on flood water levels.

However the bust was further exacerbated by the weather impacts in the area, contributing to the greater mortality rates. Other species found in the river include Murray Cod, Golden Perch, Silver Perch and Carp.

The heatwaves have put additional stress on a system that was just recovering from severe flooding. The triple La Nina that has caused enhanced rainfall in the region over the last three years has finally come to an end.