The Australian Conservation Foundation says a planned new exploration tunnel in the Kakadu National Park is a sign that ERA is expanding its operations at the Ranger Uranium Mine.

The foundation's Dave Sweeney says the Federal Environment Minister, Peter Garrett, approved ERA's application to build a three kilometre exploration tunnel under the Magela Creek without proper environmental assessment.

"It's extraordinary that a leaking uranium mine inside our largest national park and a World Heritage listed area can dig a two to three kilometre tunnel.

"And that's not seen as worthy of public scrutiny and independent assessment."

He says the application may be labelled as exploration, but the tunnel is really the first step in a new uranium mining enterprise.

"It is a key piece of mining infrastructure and they will be extracting, stockpiling and potentially processing mineralised ore from this.

"So for the Government to say that it doesn't constitute a threat to Kakadu, it doesn't constitute something that requires rigorous assesment, defies belief."

A spokesman for the Minister says a delegate of the Minister determined that the proposal did not need approval under national environment law.

The spokesman says exploration does not mean mining will automatically follow.

He says if commercial mining were to be contemplated by ERA, the department would expect that proposal to be referred and examined under the Environment Protection and Biodiversity Conservation Act.