Energy Resources Australia (ERA) says its move to underground mining through an exploration tunnel near Kakadu National Park is on schedule.

The company began digging the two-kilometre tunnel in May, with the aim of extending the operational life of the Ranger uranium mine.

ERA chief executive Rob Atkinson says the box cut of the tunnel is nearly finished.

"We are starting the very exciting work of starting the arches," he said.

"We will actually be going underground within the next couple of months, so I am very pleased with the progress."

ERA says concerns that it might start mining rock and uranium from the tunnel before seeking official approval are unfounded.

The company was given permission to construct the exploration tunnel near the open-cut Ranger mine.

Environmental groups are worried it could start extracting tonnes of rock and uranium ore from the tunnel without going through the proper environmental channels.

Mr Atkinson says the company has flagged some future possibilities of doing so with interested parties and it will seek official approval if it decides to go ahead with them.

"These things take multiple years to do," he said.

"It is part and parcel of mining processes that, rather than waiting until the last moment to do things, we let our key stakeholders, the decision makers, know what our thinking is.

"Certainly, we have still got to seek approval for that (ore extraction) and we will got through all the normal channels, and be very, very transparent about it."