Hundreds of thousands of fish have died in outback Australia, with the government blaming the mass death on a toxic algae bloom due to changing weather conditions.

It is the second incident of mass fish death along the Darling River at Menindee, around 1,000km west of Sydney, in recent weeks.

In December, 10,000 fish died in another mass fish kill.

The New South Wales Department of Primary Industries said that a sudden cooling of the temperature over the weekend killed a pre-existing algae bloom in the area, resulting in the decomposing algae draining oxygen from the river and killing the fish.

The Australian Broadcasting Corporation reported that native species including bony bream, perch and Murray cod were among those found floating on the surface and washed ashore on the river banks at Menindee.