Energy Resources Australia says open-cut uranium mining has been completed at its Ranger mine in the Northern Territory after more than 30 years.

Open-cut mining started at the site in 1980, making it one of the world's longest continually producing uranium mines.

The mine is near Jabiru, within the borders of Kakadu National Park, about 250 kilometres south-east of Darwin.

About 30 million tonnes of material will be backfilled into the pit over the next two years.

It is expected that exploration drilling for the mine's proposed new underground operation, known as Ranger Three Deep, will begin in the region next year.

ERA says it will have to put in years of work to secure environmental approval and traditional owner consent for underground mining.

General manager of operations, Tim Eckersley, says the company has started digging an exploration decline tunnel.

"Thus far, we have approval to undertake the decline as an exploration decline," he said.

"We would be expecting the environmental approvals in 2015, so there's several years of work there, along with our agreements with traditional owners, before we would begin to undertake any mining underground."

ERA plans to reinject concentrated brine from the Ranger tailings dam into the open cut mine pit they have started to refill.

It is building a brine concentrator to boil off water from the tailings dam.

Mr Eckersley says the $220 million dollar concentrator will return salty water to the dam for the next few years.

"The aim, longer term, is that we will reinject it to the bottom of this rehabilitated Pit Three," he said.

"Because salty water is heavier than fresh water it will sit in the rock-fill in the bottom of the pit."