Large-scale fish deaths have been found this week in the Darling River in the New South Wales' town of Menindee, Australia.

The Department of Primary Industries in New South Wales state said the fish deaths coincided with a heat wave that put stress on a system that has experienced extreme conditions from wide-scale flooding.

The deaths were likely caused by low oxygen levels as floods recede, a situation made worse by fish needing more oxygen because of the warmer weather, the department said.

The state planning and environment agency warned river oxygen levels could fall further this weekend as temperatures rise, before cooler conditions return next week.

Residents of the Outback town of Menindee complained of a terrible smell from the dead fish.

It follows fish deaths in the same area in 2018 and 2019, when up to a million fish died from poor water flow, poor water quality and sudden temperature changes.

(With input from agencies, cover image a screenshot)

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