Three weeks after an unprecedented fish kill in NSW's far west, the state government has confirmed a second wave of deaths with fears the situation could get worse.

Regional Water Minister Niall Blair, who is expected to visit the Menindee region on Tuesday, described the latest fish kill as an "environmental catastrophe" on Monday.

He warned that conditions in the Darling River could deteriorate further.

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"The conditions out there are showing a lot of fish that are near the surface of the water, gasping for air," he told reporters in Sydney.

Reports were also received of fish deaths in Lake Keepit and Thredbo.

Up to a million fish died along the Darling River at Menindee earlier in January, while thousands were also found dead almost 900km away along the Macintyre River.