The banks of the Murray-Darling Rivers are thick with rotten fish, with officials putting the number of dead at hundreds of thousands and saying the toll is likely closer to one million.

Further high temperatures forecast for this week could make the situation worse, the New South Wales government has warned.

Low water conditions and the heat may also have encouraged an algae bloom that starves the fish of oxygen and produce toxins.

"We do expect to see more fish kills across parts of the far west and Northern Tablelands this week," said state minister Niall Blair.

The deaths have become a national issue, sparking angry allegations about the cause and who is responsible.

"It's a devastating ecological event," said Prime Minister Scott Morrison on Monday, pointing to apocalyptic scenes. "The sheer visual image of this is just terribly upsetting," he said.

Morrison's government has blamed the fish deaths on drought, and defended policies which some locals say has caused the systemic depletion and pollution of the river system.

"There's a drought and this is one of the consequences of drought. There are many, and my focus on drought has not shifted one inch," Morrison said.

But for years scientists have been warning of people extracting vast amounts of water without check for irrigation or other uses, undercutting billions of dollars of investment.

"Dead fish and dying rivers are not because of the drought, it's because we are extracting too much water from our river," said John Williams, an expert in water economics at the Australian National University.

Opposition leader Bill Shorten called on the government to set up an "emergency task force" to address the issue.

"You can't ignore a million dead fish, that's a shocking development," said Shorten.

Morrison insisted the management plan for the Murray-Darling Basin was bipartisan, adding that his government was only following on the policies of the opposition Labor government.

"I'm concerned today that some might want to play politics with that," he said.

"There were reports done by scientists under Labor's contribution to that plan back in 2012 and the plan has been operating in accordance with that advice."

Scientists are calling for politicians to be held to account.

"Billions have been spent on upgrading irrigation infrastructure," said Quentin Grafton, also of the Australian National University, "but with no public benefit."

"It's a disgrace and it's time those responsible are held accountable for this unfolding disaster."