Millions of litres of radioactive water from the Ranger uranium mine have flowed into world heritage-listed wetlands in Kakadu National Park.

Aboriginal traditional owners say they will oppose plans for a huge expansion of the 30-year-old mine by Energy Resources of Australia unless the company upgrades its environmental protection procedures.

The Rio Tinto-owned company has attempted to downplay an unexplained spike in contaminated water flowing from the mine into Kakadu's Magela Creek between April 9 and 11, the Heraldcan reveal.

About 40 Aboriginal people live downstream from a site where a measure probe recorded up to five times the warning level of electrical conductivity, which is a measure of contaminants including uranium, sulphate and radium.

The environmental group Environment Centre NT has been leaked evidence detailing the spike, which ERA representatives claimed had originated upstream from the mine and was not the company's fault.

Asked by the Herald about the contamination, ERA admitted the source of it "could not be determined and investigations are continuing".

"It is possible that these have come from the Ranger operations," the company said.

ERA's handling of the spike and other environmental concerns have strained relations with the Gundjeihmi Aboriginal Corporation, which represents the Mirarr traditional owners.

In another unreported mishap at the mine in December, a poorly engineered dam collapsed, spilling 6 million litres of radioactive water into the Gulungul Creek, which flows into Kakadu.

Asked about the collapse, ERA acknowledged that the dam had been poorly constructed and said engineering and construction standards had been put in place to ensure it never happened again.

Scientists confirmed last year that the mine's tailings dam was leaking 100,000 litres of radioactive water into the earth and rock fissures below Kakadu every day.

ERA has denied any serious seepage from the dam, which contains 10 million litres of radioactive water.

Yvonne Margarula, a senior Mirarr elder, said the company told her people "only the good stuff … they don't tell us the whole story. [They] treat us mob like something else, like we don't know, like kids."

Ms Margarula said she worried about Aboriginal children swimming and fishing in the polluted creeks.

She said that Mirarr were not respected by the company.

Justin O'Brien, the Gundjeihmi Corporation's executive officer, said the traditional owners had decided to make their concerns public despite having good relations with the company on other issues, including renegotiation of the mine agreement and settlement of a native title claim over the mining town of Jabiru.

He said unless the company changed its environmental procedures, the Mirarr would not support any expansion of the mine, which included a heap leaching plant, a tunnel under flood plains, a 1000-person accommodation village, 650 evaporation ponds and a one-square-kilometre tailings dam.

The mine, which was originally scheduled to close in 2008, has had more than 150 spills, leaks and licence breaches since it opened in 1981.

David Paterson, ERA's general manager, external relations, said the company took the concerns of the Mirarr people seriously. He said it had regular communications with the traditional owners and the Northern Land Council.