Contaminated water seeping from a mine in Kakadu National Park has a uranium concentration more than 5,000 times the normal level, a Senate estimates committee has heard.

The Office of the Supervising Scientist is the environmental regulator of the Ranger uranium mine, which is owned by Energy Resources of Australia.

The office today told the committee that water seeping from underneath the dam has about 5,400 times the level of uranium than the natural background level.

Greens Senator Scott Ludlam says the environmental regulator told the committee about 100,000 litres of water seeps from the tailings dam every day.

Mr Ludlam says the water has been leaking from the dam for years.

He says the regulator says it will be impossible to rehabilitate the site.

"The uranium concentation in the billabong surrounding the mine are about three to five parts per billion," he said.

"But the uranium in the processed water that is leaking from beneath the tailings dam is 27,000 parts per billion.

"So it's roughly 5,500 times as much uramium in that water as there is the surrounding environment and that means the company has got a huge problem."

He says ERA must make clear the effect it is having on the nearby environment.

"ERA made a quarter of a billion dollars in profit last year," he said.

"I think the least they can do for the cultural and environmental integrity of Kakadu National Park is do some proper water quality sampling so we know what kind of contamination we are dealing with."

ERA says it cannot confirm the uranium concentration figure because it says it does not know the context in which it was reported.

The company refutes that 100,000 litres a day are leaking from the dam.

It says the Ranger uranium mine is one of the most environmentally regulated mines in the world.