Thousands of fish have been killed in a stretch of the Murray River at Thologolong, including Murray cod weighing an estimated 40 kilograms.

The cod, some over a metre in length, were discovered in the water and banks off Murray River Road, about 150km from Albury, on Wednesday and Thursday.

Thousands of dead fish including cod, carp, a small number of redfin, Murray crayfish, shrimp and small fish have washed up on the rocks.

Rainfall is thought to have washed ash and charcoal into the water, causing it to lose oxygen and suffocate the fish.

Peter Sutherland picked up Murray cod at his property with Cameron Howard on Thursday morning and said the situation would likely get worse.

He is concerned dead carp will continue to pollute the water, which he said a North East Water employee told him was unsafe to drink.

"I've seen the odd fish kills, but nothing like this," the 78-year-old said.

"It's a disaster in my opinion. The water has had charcoal from Jingellic and Cudgewa wash into it.

"A lack of oxygen and anything in the river will die. They've got no hope."

The water is clearly blackened and charcoal is visible on the banks.

Dead fish could be seen floating down the river on Thursday.

Cameron Howard has been working on Mr Sutherland's farm and was concerned the water could kill stock.

"More deaths are likely to occur," he said. "There's about 30 carp to each cod.

"It stinks. If you get a hot day it will be horrible."

It's unclear how old the larger cod were, but Mr Sutherland thought they could be 30 or 40 years old.

There are concerns the loss of juvenile fish, of which thousands are thought to have been killed, may impact the ecosystem.

Mr Sutherland feared authorities had been too slow to act after the incident.

It had been left to nearby residents to try to remove fish from the river, he said.

"I did warn a week ago this would happen, but nobody listened to me until today," he said.

Mr Sutherland said there were similar issues with fish deaths following the 1952 fires in the region.