According to locals, this is the largest fish death event to hit the town that just witnessed another significant mass fish death three years earlier.

Residents in the regional Australian town of Menindee on Friday woke to the discovery of millions of dead fish in their river.

The large-scale fish deaths were first recorded on Friday morning in the town of about 500 population located in the far-western state of New South Wales (NSW).

The state’s river authority said it was caused by an ongoing heat wave that affected the Darling-Baaka river.

The NSW Department of Primary Industries (DPI) said the heat wave put “further stress on a system that has experienced extreme conditions from wide-scale flooding”.

Due to human-caused climate change, heat waves have become more frequent, more intense, and last longer. The world has already warmed by around 1.1 degrees Celsius since the industrial age began, and temperatures will keep rising unless governments around the world drastically reduce emissions.

Menindee resident Graeme McCrabb described the fatalities in an interview with BBC as “surreal”, saying: “It’ll probably be a bit more confronting today.”

He added that locals in the town rely on the Darling-Baaka for water supplies and warned that they were anticipating that even more fish would die as the already decomposing fish sucked more oxygen from the water.

The NSW DPI also said the fish deaths were “distressing to the local community,” a sentiment McCrabb echoed.

On Saturday, the temperature in Menindee is anticipated to reach 41 degrees Celsius.

The Darling-Baaka river is part of the Murray Darling Basin, Australia’s largest river system. This week’s fish deaths throw light on the problems facing the Basin. Drought and excessive human use have impacted the Murray Darling ecosystem’s health.

The Murray Darling Basin authority said agriculture, industries, and communities have used water from the river system, resulting in less water flowing through the river. It also said the Basin is prone to extreme weather events and has a highly fluctuating climate that makes it vulnerable to both fires and droughts.

A plan worth A$13bn (£8.45bn at the time) was implemented in 2012 to try and stop the river from drying up and returning it to a healthier level.

The NSW DPI said it will work with federal agencies to respond to the latest incident and to find the underlying causes of the deaths.