A mass fish kill in NSW has sparked anger.

Locals are reporting that the carcasses of thousands of fish now line the shores of Lake Wyangan in Griffith, including dead Murray Cod, yellow belly perch, bony bream and carp.

Griffith City Mayor John Dal Broi says the incident is being investigated by the NSW Environment Protection Authority and Department of Primary Industries.

“We're testing the water, we've collected a number of fish to undergo tests and we're awaiting the results,” Cr Dal Broi said.

“The fish fatalities are obviously very distressing and Council is making every endeavour to find out why this occurred.”

Shooters, Fishers and Farmers MP Helen Dalton says contaminated runoff could be to blame.

Griffith City Council has a project officer dedicated to improving the lake's water quality, but Ms Dalton says it should do more.

“They've done a lot of talking, that's for sure,” she said.

“We have a resident in Griffith, Geoff Sainty, who's a world-leading expert on wetlands, but the Mayor and general manager refuse to bring him on board.

“He knows there's a solution and yes, it's going to take a little bit of time and money, but the lake is an asset to the community and it's now a health hazard.

“It needs to be fixed as soon as possible.”

The council says it has already heard those claims.

“A series of recommendations to council tomorrow night follow through on Mr Sainty's ideas,” the Mayor said.

The council also has plans for a new pipeline to improve circulation of water through the lake, and measure to revegetate areas where water enters the lake to improve filtering.