Aboriginal traditional owners will oppose plans to expand the Ranger uranium mine in Kakadu unless the operating company improves its environmental record.

The Gundjeihmi Aboriginal Corporation released a statement on behalf of Mirarr traditional owners on Monday condemning the "poor environmental performance" of Energy Resources of Australia (ERA), after it was revealed contaminated water had flowed from the mine into Kakadu between April 9 and April 11.

Mirarr traditional owners are calling on ERA to enter into a new partnership to protect the world heritage-listed national park from environmental damage.

"The Mirarr have stressed that they will oppose the proposed expansion of the mine, including a proposed heap leach facility and the extension of mining in the so-called Ranger 3 Deeps area, until ERA can genuinely and demonstrably improve its environmental performance," the statement said.

Mirarr senior traditional owner Yvonne Margarula said Aborigines were worried about the ongoing environmental impacts of the leak on Magela Creek, downstream from the mine.

"It is the biggest worry, because all the kids, they go there fishing and then swimming," she said, adding that she was concerned ERA informed traditional owners only about the "good stuff".

Gundjeihmi Aboriginal Corporation executive officer Justin O'Brien said that in many respects the relationship with the mining company had never been better, but that the entire relationship was threatened by ERA's ongoing poor environmental performance.

"A new and transparent partnership between Mirarr and the company is required to honestly address these issues," he said in the statement.