THE operators of the Ranger Uranium Mine in Kakadu have been accused of trying to expand the mine by stealth after Environment Minister Peter Garrett approved a three-kilometre exploration tunnel.

The decision to approve the tunnel with no environmental conditions has angered environmentalists, who say it is the first step towards expanding the 30-year old mine, situated in the world heritage-protected Kakadu National Park.

The decision was quietly released on Sunday.

The tunnel will reach a depth of up to 350 metres underground and travel outside the existing environmental impact zone of the mine and under the important Magela Creek, which brings water to Kakadu's wetlands.

Nuclear campaigner for the Australian Conservation Foundation Dave Sweeney says details found in mine operator Energy Resources of Australia's proposal for the tunnel showed it was expansive enough to be later used or modified for commercial mining. The proposal has provision for the tunnel to follow any seams of uranium surveyors come across.

The federal Environment Department assessed the tunnel as an "exploration project" under the Environmental Protection and Biodiversity Conservation Act - and did not consider it as an expansion of mining.

But Mr Sweeney said the decision not to further scrutinise the tunnel means that once it is built it will be harder to reject expansion plans when a proposal comes before Mr Garrett.

The mine was due to close last year, but the company now hopes to continue extracting uranium until 2021.

A spokeswoman for ERA said it was "too hypothetical" to comment on whether the tunnel could be later used for commercial mining operations.

A spokesman for Mr Garrett said exploration did not mean mining automatically followed.

"If commercial mining were to be contemplated by ERA then we would expect that proposal to be referred and examined under the EPBC Act," the spokesman said.

It was "likely that a high level of assessment would be required for any proposal to expand mining at the Ranger uranium mine", he said.

Construction of the tunnel is due to begin in mid-2010.

Another proposal by ERA for a "heaped leech facility" - in which low grade uranium in rocks is melted down then processed - has been referred to the Environment Department for further scrutiny.

The Ranger mine has a long history of controversy, including revelations last month that 100,000 litres of contaminated water leaked from the site every day.

In 2005 ERA was fined for accidentally exposing its employees to low-level radiation when water that workers used to drink and wash with was contaminated. It was one of 150 estimated environmental incidents since the mine began operating in 1980.

Mr Sweeney, a veteran campaigner against the mine, said the long history of condemnations and leaks meant that rather than expanding the mine it should be shut down as soon as possible.

"The Ranger is like a middle-aged man," he said. "It's ageing, leaking and expanding."