Authorities investigate a big fish kill in north-west Victoria - 200,000 may have died

The north end of Kangaroo Lake, between Kerang and Swan Hill, was carpeted with dead fish, some more than a metre long.

Carp, bony bream, yellow belly and Murray cod are among an estimated 200,000 fish found dead along the shore.

Testing of the lake's water has today confirmed it was deoxygenated.

Both the EPA and Goulburn Murray Water are investigating the incident.

GMW said it was "limited in its ability" to prevent more fish deaths.

GMW water delivery services general manager Warren Blyth said deoxygenated water can occur naturally, often due to changes in temperature.

It reached a maximum temperature of 41 degrees at Kerang on Saturday.

"The impact of floodwaters is still prevalent in the area," he said.

GMW issued a blue-green algae warning for the popular recreation lake, and the adjacent Racecourse Lake, last month.

The lake, adjacent to the Murray Valley Highway, is one of the largest and deepest permanent freshwater lakes supplied by the Torrumbarry Irrigation System in the north-west of the state.

It is 12km wide.

Mr Blyth said the algae contributes to the water's deoxygenation.

"We are unable to confirm the exact number of fish deaths, but believe this is mainly European carp, an invasive pest species in Australia," he said.

"Unfortunately, golden perch and cod have been affected by this event.

"We will monitor the location for further fish deaths, and will continue collecting water samples to test for any further changes.

"We will also conduct a cleanup of the dead fish at the lake."

Late last year it was a blackwater event triggered by floods which caused fish kills and Murray crayfish leaving a long stretch of the Murray River from Echuca downstream to Swan Hill.

Blackwater floods are said to be natural events which often happen in flood time which removes oxygen from the water.

Murray-Darling Basin Authority senior director, environmental management, Dr Janet Pritchard said hypoxic blackwater events occur as leaf litter and other carbon-based debris is swept into waterways.

"As this material is consumed and broken down by bacteria, oxygen can be sucked out of the water, making it difficult for fish and other aquatic animals to survive," Dr Pritchard said.

Meanwhile, national attention is focussed on another massive fish kill at Menindee in the lower Darling-Bakaa River south-east of Broken Hill.

Millions of fish, including golden perch and cod, have been killed due to hypoxic blackwater event.

"It's potent ... the smell, it's hard to find a word for it, it's just putrid," Menindee Lakes resident Graeme McCrabb said.

Mr McCrabb, who lives downstream of Menindee, local residents were no strangers to fish kill incidents with many occurring over the years.

But he said this would have to be the worst.

There were three significant fish kills in 2018/19 and there have been several smaller events since then.