His trout are dying in droves - he estimates about 100 in the past couple of weeks - and he expects it will take the fish up to a month to recover their numbers.

Financially, he faces an even longer period of strain, with the loss expected to take a significant toll on the business until about June this year.

"They are suffering .... [and] we're losing money and eating into our savings. It's hitting us hard," he said.

"I would certainly hope no more fish will die but you can always expect some [deaths during a heatwave] ... when water heats up, it takes a long time to bring the temperature back down again.

The Snowy Mountains trout farm, like much of Australia, has been in the midst of an extreme heatwave this week, with temperatures reaching into the high 30s from Monday.

This, in turn, has seen a dramatic increase in water temperatures. Eucumbene Trout Farm's waters are measuring at 20 degrees and beyond, when ideal temperatures for trout are between 5 and 15 degrees.

With the increase in temperatures comes a lack of oxygen in the water. Trout, in response, either "shut down" and lay on the bottom of river banks as a means of survival, or suffocate and die.

"These temperatures are not favourable for trout by any stretch ... but it's all about the amount of oxygen in the water," Mr Cottrell said.

"We're pumping a lot of oxygen into the water so we can keep the fish alive but we have some casualties. Without us pumping oxygen in, the water temperature reaches 22 degrees."

Eucumbene Trout Farm isn't alone in its experience of mass tragedy, with the recent Murray-Darling disaster being attributed in part to hot, dry, conditions in southeast Australia, and the blue-green algal bloom.

Preliminary figures from a report being prepared by the NSW Department of Primary Industries put the number of dead fish from the disaster at about 100,000.

Several other waterways across the state are suffering, too. This includes the Lower Murrumbidgee River, from Hay to Balranald, which is currently on "red alert" as a result of the heat-induced bacterial bloom.

The NSW Department of Primary Industries says it anticipates more fish kills in the coming months as the algae continues to deprive waterways, and fish, of oxygen.

"With ongoing extreme heat and drought conditions expected to continue over the coming months it is likely more fish kills will occur without significant rainfall to generate replenishment flows," a spokesperson said.

Mr Cottrell says that, while he will continue to manage his private stock of about 3500 to 4000 trout using manual aeration, the proposal to use the same solution to fix the Murray-Darling disaster is "beyond comprehension".

"We have 100,000 litre [oxygen] tanks, and and we're a dwarf and they're a Hercules or something. It would be millions of millions of litres," he said.

The Eucumbene Trout Farm operates out of a 25 million litre, or 0.03 GL dam. The total capacity of all catchments and dams in the Murray-Darling Basin is 22,256 GL. A gigalitre is one billion litres.

Temperatures around the Eucumbene Trout Farm are expected to drop by between 5 and 10 degrees on Friday night, with a southerly change expected to see relief across most of southeast NSW and the ACT.

The drop in air temperature will eventuate in a drop of water temperature, too, but the slowed, gradual cooling to an ideal temperature of 5 to 15 degrees will take weeks.

The NSW Department of Primary Industries said environmental water is currently being released into the lower sections of the Murrumbidgee River to help maintain water quality.