There is no guarantee that one of world's largest uranium mines will be expanded, despite the discovery of fresh deposits, the Northern Territory government says.

Energy Resources of Australia (ERA) announced on Monday that it had found a potential further 40,000 tonnes of uranium oxide near the Ranger mine, about 260km east of Darwin.

The discovery could extend by seven years the life of the mine, which is surrounded by Kakadu National Park.

It already produces 11 per cent of the world's uranium.

"The options are an underground operation, and perhaps an option is an extension of the current open cut mine," the company's chief executive Rob Atkinson said on Monday.

But NT Mines Minister Kon Vatskalis said there were no guarantees the mine would be expanded.

"Any mine in the Territory to expand has to undergo a very strong assessment by our government departments, Mines and Environment and in the case of ERA from the Commonwealth Departments," he told ABC Radio.

"It's not just a fait accompli ... If it happens, it has to be assessed very strictly against the current environmental and mining legislation."

David Paterson, general manager of ERA business development, conceded that any expansion was a long way off.

"All those studies need to be done," he said.

"Mining is due (at present) to be completed in 2012 and processing continues out to 2020."

Dave Sweeney, from the Australian Conservation Foundation, warned the environmental impact was already considerable.

"(This is) a company that has had some spectacular mistakes," he said.

"Workers ended up drinking and showering in water that had greatly elevated levels of uranium contamination."

Mr Sweeney said ERA was required to "clean up its mess".

"It would increase the range of risks and the complexity of the clean-up," he said of the expansion.

But Mr Paterson said ERA had learned from its mistakes.

"There clearly have been some incidents in the past which are regrettable and they have been dealt with fully and they have changed the way we operate," he said.