Farmers along the Darling River are resorting to using sulphuric acid to counter high salt levels as attention turns to the quality of the water that remains in the drought-hit system.

One in three native fish species in the NSW section of the Murray Darling Basin is threatened with extinction, according to the State of Environment report released earlier this month. However, the situation is likely to be worse since the report was based on a 2017 audit of the system by the Basin Authority, before the massive fish kills of the past summer.

The 2017 audit described the Darling's condition was described as "moderate". Sampling during the winter months by fisheries would determine if the endangered status of native species has worsened since then, a spokeswoman for the Department of Primary Industries' fisheries unit said.

Carcasses after a mass fish kill in the Darling River at Menindee in January. Credit:Nick Moir

Residents are expecting the health of the river to be officially downgraded. "A few million fish killed here raises questions about the status being 'moderate'," said Graeme McCrabb, a resident of the far-western NSW town of Menindee.