Environmentalists are calling for the Federal Government to reject a proposal to expand the Ranger uranium mine, which lies within the boundaries of Kakadu National Park.

The company ERA has put forward plans to expand and also use an acid leaching method to retrieve uranium and other minerals.

Australian Conservation Foundation (ACF) spokesman Dave Sweeney says news that contaminated water is leaking from the mine's tailings dam is proof the company cannot properly treat its waste.

"Ranger is failing now already," he said.

"The last thing that we need is to expand that mine, expand the problems, expand the pollution.

"This mine should be being cleaned up and being phased out, certainly not expanding."

ERA's general manger of business development, David Patterson, says the leakage is not seeping into the park and the company has strict waste management regulations that are working.

"The office of the supervising scientist is well aware of the issue that they've raised and has confirmed each year that ERA has no impact on the surrounding environment," he said.