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Environmentalists and farmers blamed the management of Menindee Lakes by the Murray Darling Basin Authority for the fish kills in 2019.

That fish kill was not caused by a black water event, but rather a build-up of blue-green algae that is estimated to have claimed one million native fish.

The lakes were filled with floodwaters by 2017, much of which was released into the Lower Darling. Locals argued water should have been held back to ensure a fresh supply was available to replenish the Lower Darling downstream.

Drought-hit the region and by 2019 blue-green algae flourished in the stagnant pools downstream of Menindee Lakes.

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When a sudden cold front came through, dropping the temperature by 15 degrees, the algae died and bacteria that break down algae went to work eating the dead organic matter. As the bacteria went to work it chewed up the oxygen in the water, creating a hypoxic environment that suffocated the fish stuck in isolated river pools.

A spokesperson for the NSW Environment Protection Authority said the agency was investigating the Parramatta River fish kills, and “preliminary investigations” suggested the deaths were a result of “high-intensity storms that washed organic matter into the waterway, which combined with elevated water temperatures, has caused the fish deaths”.

“It is distressing to see a significant number of fish killed and a variety of species impacted,” the spokesperson said.

Dr Ian Wright, who lectures in water science and management at Western Sydney University, said the Parramatta fish kills are an “enormous concern” and showed how fragile the environment was in urban areas.