Workers from the controversial Ranger mine in the Northern Territory have been contaminated with uranium, a union says.

Worksafe has been called in to investigate the claims relating to the Energy Resources of Australia (ERA) mine, about 260km east of Darwin.

Australian Manufacturing Workers Union (AMWU) spokesman Steve Milne said nine workers were asked to clean up a yellow cake spill three weeks ago.

Since then, some of the workers had tested positive for dangerous levels of uranium in their urine, he said.

"Some of them are on the lower end," he told ABC Radio.

"(But) one, I believe is about three times the acceptable standards of uranium exposure.

"Mind you, from our point of view, any form of exposure to uranium is a danger because uranium basically has some major consequences on a person's ongoing health."

Mr Milne said a vacuum machine could have been used instead of the workers to clean up the spill.

Dave Sweeney, from the Australian Conservation Foundation, said he had heard reports that up to 15 people had been exposed to elevated levels of uranium oxide.

"Workers dressed only in paper overalls and without the required monitoring devices were literally caked in uranium during the incident last month," he said in a statement.

"This is the latest in a long history of environmental and occupational leaks, spills, incidents and accidents."

But ERA business development manager David Paterson said any exposure to the uranium was within safe levels.

"All of their exposure was well below any safe levels of exposure to radiation," he told the ABC.

Earlier this week ERA announced it had found a potential further 40,000 tonnes of uranium oxide near the Ranger mine which could extend its life by seven years.

Surrounded by Kakadu National Park, it already produces 11 per cent of the world's uranium.

Mr Sweeney said the latest reports showed the mine was not run responsibly and it should not be allowed to expand its operation.

"This would be a massive burden on the Kakadu environment and cause massive long term pollution problems," he said.

"Today's news again shows that the company cuts corners and compromises environmental and human health."

A 2003 Senate Committee report found there had been 110 pollution incidents and numerous breaches of environmental requirements at the Ranger mine since 1981.