Surviving fish in the lower Darling River will be tested by NSW authorities to determine the impact two recent mass deaths had on the population.

Millions of fish were killed in a 40 kilometre stretch of the Darling River near Menindee in December and January, with an expert panel of scientists attributing the loss to a lack of water in the Murray-Darling river system due to drought and excess use by irrigators.

The NSW Department of Primary Industries on Monday announced it will sample the remaining fish populations in the lower Darling River next week.

"This critical research will allow us to determine the extent of the impact from the fish kill events; report an estimate on the number of fish that survived and their current condition, and guide management actions in the region to protect and recover native fish populations," Sarah Fairfull from DPI Fisheries said in a statement.

"More than 500km of the lower Darling was unaffected by these fish kill events, which provides DPI Fisheries scientists with the ability to compare populations and to support the restoration of the affected reach."