Officials from the Department of Primary Industries in New South Wales state said the fish die-off was a phenomenon that coincided with a heat wave.

The high temperatures have caused stress to the ecosystem, which is under pressure from extreme weather conditions, among other things, due to major floods.

The deaths of large numbers of fish were likely caused by very low oxygen levels with the water receding. The whole situation worsens because the fish need more oxygen during warmer weather conditions to function normally.

Mass die-offs have been reported on the Darling-Baka River in recent weeks.

Tens of thousands of fish were found in the same location at the end of February, and a number of dead fish were reported downstream towards Punkari, not far from the border of South Australia and the state of Victoria.