The company which operates the Ranger uranium mine in the Northern Territory says it has potentially found massive additional amounts of uranium oxide.

In a statement to the Australian Stock Exchange, Energy Resources of Australia (ERA) announced it believes an area adjacent to the current mine could contain up to 40,000 tonnes of uranium oxide.

Ranger is already one of the biggest uranium mines in the world.

It is surrounded by Kakadu National Park and produces 11 percent of the world's uranium.

The company's chief executive, Rob Atkinson says he cannot yet put a potential value on the deposit, but it will probably significantly extend the life of the mine.

"On world standards to give you an indication currently at the Ranger mine we are producing between 5,000 and 6,000 tonnes a year, so that (the discovery) would potentially extend the life or give us optionality for another six, seven years," he said.

Mr Atkinson says the company is not yet sure how future mining will proceed.

"The options are an underground operation, and perhaps an option is an extension of the current open cut mine."

The Australian Conservation Foundation says it is concerned about the environmental impact of more uranium mining in the Kakadu region.

A spokesman for the Conservation Foundation, Dave Sweeney says he is concerned about accidental spills and contamination.

"It's a company that has had some spectacular mistakes, including one a few years ago that many people will remember where the wrong pipe was connected to the wrong pump and workers ended up drinking and showering in water that had greatly elevated levels of uranium contamination," Mr Sweeney said.

"It would increase the range of risks and the complexity of the clean-up.

"Will people ever see their country cleaned up and rehabilitated and returned into Kakadu and protected? For many Aboriginal people in the region and for many traditional owners, that would be a really serious burden and a really serious disappointment today," he said.