The major fish kill northwest of Griffith was reported by locals on Saturday and included thousands of dead native species, including the murray cod, yellow belly perch and bony bream, washed up on the shores of the recreational lake.

NSW Murray MP Helen Dalton visited the lake on Monday and found it to be "absolutely putrid, dark in colour, full of foam and sludge with dead native fish floating up on the shore".

"This is a threat to human life, not just marine life," she said in a statement.

The Shooters, Fishers and Farmers MP compared the situation with that of the Menindee fish kill where an estimated one million fish died in January 2019.

"Over this past year, I've written urgent letters to NSW government departments urging action on Lake Wyangan and they've done nothing," Ms Dalton said.

"We have about 20 government departments that are supposed to look after our waterways but none of them seem to give a damn about water quality.

"This fish kill must be a final wake-up call for all levels of government to protect our most valuable natural resource."

Lake Wyangan was originally a quarry and has accumulated large quantities of nutrient-loaded sediment over the years.

It has also been plagued by outbreaks of toxic blue-green algae.

Griffith City Council said on Monday the incident was reported immediately to the NSW Environmental Protection Agency and NSW Department of Primary Industries.

Council general manager Brett Stonestreet said water samples along with samples from each fish species had been sent away for testing and discussions would soon take place between council and the DPI.

Mr Stonestreet said it was too early to speculate on what caused the fish fatalities but Ms Dalton said she suspected contaminated runoff could be to blame.

About 240ML of water was piped into Lake Wyangan North in early May via the southern inlet while additional water also came into the lake with the recent rainfall, a Griffith City Council release stated.

Griffith Mayor John Dal Broi said the council allocates 1500ML of water from its annual licence to assist with replenishing water losses at the lake.

"Council made the decision that to put large quantities of water into Lake Wyangan during a time of extreme drought would be irresponsible and instead decided to allow water levels to reduce," Cr Dal Broi said in a statement.

"The fish fatalities are obviously very distressing and council is making every endeavour to find out why this occurred."

Cr Dal Broi said the council is working to control sediment inflow and improve water quality at the lake through the construction of a series of sedimentation ponds and wetlands while also trying to discourage the growth of blue-green algae through circulation of the water.

Australian Associated Press