The operators of the Ranger mine in the Northern Territory say a spill of uranium and acid has been contained, and there will be no impact to the environment.

A huge tank in the processing area of the mine failed about 1:00am (ACST) on Saturday.

The tank containing radioactive material burst open and its contents flowed outside the banks meant to keep any leaks contained.

As much as 1,000 cubic metres of slurry was spilled at the mine site near Jabiru.

Workers had discovered a hole in the side of the tank and were evacuated before the tank burst and the slurry escaped.

The mine's operator, Energy Resources of Australia (ERA), says no-one was injured and no uranium leaked off the site into the surrounding Kakadu National Park.

But traditional owner groups say they are "sick with worry" about the potential environment impact.

Photos of the site taken by the Gundjeihmi Aboriginal Corporation and supplied to the ABC suggest material did spill onto grassy ground at the site.

ERA spokesman Tim Eckersley says there is no environmental emergency.

"They evacuated the area and at about 1:00am the tank basically split at the bottom and the processing slurry, which is a mixture of mud and water, has spilled out the bottom of the tank.

"That's the beginning of our processing operations, so it's a mixture of ground-up uranium ore and acid."

He said the material mostly spilled onto compacted earth, tarmac and drains.

"It's very impervious material so there's very little chance of it leaking into the soil there," Mr Eckersley added.

The company said earlier in a statement the slurry moved outside the containment area, but was captured and contained on-site.

"As the material was contained within the processing area there is no impact on the environment surrounding the Ranger project area," the statement said.

Investigation begins as anti-nuclear campaigners slam company

Federal Environment Minister Greg Hunt has ordered an immediate clean-up and investigation.

A spokesman for the minister said the leak has been contained and will have no impact on Kakadu National Park.

But the Northern Territory Environment Centre is calling for an immediate halt to operations at the mine.

The centre estimates around 1 million litres of acidic radioactive material spilled from the processing tank.

Anti-nuclear campaigner Lauren Mellor says it is the third safety breach by the ERA in a month.

"Just within this month we've had an incident where a controlled vehicle was able to leave a secure area of the mine and was halfway down the Arnhem Highway before it was located," she said.

"We've had four barrels found in the rural area in Darwin, four barrels used to transport uranium were discarded with no explanation."

Greens Senator Scott Ludlam has called for an indefinite suspension of operations at the Ranger mine.

"It is hard to imagine a worse time for Environment Minister Greg Hunt to be deregulating the uranium sector and leaving it to the states and territories," he said in a statement.

"This is an industry that demands much tighter regulation as we go down the path to permanently phasing it out.

"The writing has been on the wall at Ranger for a long time. This disaster may well be the last nail in this accident-prone mine."

Traditional owners call for audit of mine

The Gundjeihmi Aboriginal Corporation (GAC), which represents the Mirarr traditional owners of the area, says this is one of the worst nuclear incidents in Australian history and has called for an audit of the site's facilities.

"People living just a few kilometres downstream from the mine don't feel safe," GAC chief executive Justin O'Brien said.

"How can we trust the assurances of a company which has repeatedly failed to safely manage this highly toxic material? What may happen next?

"It's a catastrophic failure on the part of not only the operator but also the government regulators in the Northern Territory and Canberra."

It is not yet known how long it will take for work to resume at the site.

The company has applied to the Federal Government for a large underground expansion of the mine, called the Three Deeps project.

Traditional owners have not yet given their approval.