There are reports that thousands of fish have been found dead at Griffith's Lake Wyangan since Saturday, including large Murray cod.
Griffith City Council who manage the lake stated the incident was reported immediately to the NSW Environmental Protection Agency and Department of Primary Industries.
Samples had been taken of each fish species to be analysed, along with several water samples.
Lake Wyangan has been plagued by water quality issues in recent years, with blue-green algae a common occurrence. The Lake was on a red alert for blue-green algae over the summer and is currently on an amber alert.
Griffith City Council Mayor, John Dal Broi said they were committed a solutions for the Lake.
"We're very upset about it (the fish kill), the most upsetting thing is that we don't know what's caused it," Cr Dal Broi said.
"It's really knocked us for six."
Council stated that in early May, approximately 240 megalites of water was piped into Lake Wyangan North via the southern inlet with additional water coming into the Lake with the recent rainfall.
However, in general, the Lake's water levels have remained low since 2018.
Cr Dal Broi said in the 2018/2019 water year, in consultation with stakeholders, council decided to forgo topping up the Lake with the usual 1500ML from their allocation.
Instead they put only 300ML into the Lake and the remaining 1200ML was was sold, the revenue put towards funding a long-term solution for Lake Wyangan's blue-green algae issues.
"We made the decision to allow water levels to reduce while still delivering sufficient water, as supplied by Murrumbidgee Irrigation, to the Lake in order to maintain a stable habitat for fish," Cr Dal Broi said.
On Tuesday night council will vote on a contractor to construct a series of sedimentation ponds and wetlands at Lake Wyangan North as part of council's strategy to improve water quality.
"The Lake is actually an old quarry and over the decades has accumulated large quantities of sediment loaded with nutrients," Cr Dal Broi said.
"The strategy to improve the water quality is also dependent on improving circulation of the water and discouraging the growth of blue-green algae."
Griffith Boat Club president, Scott Collis said the Lake's water level was no worse than it had been last year.
"The lake was very low over the summer and the fish were alright, and there doesn't seem to be any dead fish in the South Lake which is right next door," Mr Collis said.
"I think if the water level was the issue something would have happened before now."
Member for Murray Helen Dalton said in the past Lake Wyangan had been a very important asset for Griffith.
"There was so much going on at that lake, water skiing, sailing, rowing regattas," Ms Dalton said.
"Now it's an eye-sore and health hazard, something has to change."