Energy Resources Australia (ERA) says a new chapter is beginning for its uranium mine in the Top End to move from an open cut tounderground operation.

The first sod has been turned for a $120 million dollar exploration tunnel on the Ranger mining lease, which lies within the boundaries of Kakadu National Park.

ERA executive director Rob Atkinson says it is an important step in testing the viability of expanding the mine.

"It really is, hopefully, the start of a new chapter in our 30-year history that for the first time ... we start going underground with the Ranger operation," he said.

But traditional owners of the which is leased to ERA say their approval of the new exploration does not mean they will allow the mine to expand.

The Gundjeihmi Aboriginal Corporation represents the Mirrar traditional owners.

Executive officer Justin O'Brien says they are willing to consider the results.

"Our engagement and willingness to look at exploration results should not be construed as a blank cheque," he said.

"We are going to look at these things.

"We are practical people but there is a legal constraint on all of us determined by the rules, and they are that mining ends by 2021 and rehab by 2026." 

Mr Atkinson says ERA hopes the tunnel project will confirm there is more than 34,000 tonnes of uranium oxide that can be mined.