Fresh concerns have emerged that radioactive waste from the Ranger uranium mine has flowed into world heritage-listed wetlands in Kakadu National Park.

Aboriginal traditional owners, who claim they will oppose expansion plans by Energy Resources of Australia (ERA) unless it upgrades its environmental protection procedures, have raised concerns about alleged recurring leaks on and off for at the least the past six months.

Media reports on Monday revealed details of a spike in contaminated water flowing from the mine into Kakadu's Magela Creek between April 9 and 11 were leaked to the Environment Centre NT.

"A probe recorded up to five times the warning level of electrical conductivity, which is a measure of contaminants including uranium, sulfate and radium," Fairfax Newspapers said.

ERA reportedly told Fairfax the source of the contamination could not be determined and investigations were ongoing, but that they initially believed the spill originated upstream from the mine.

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

Australian Conservation Foundation nuclear free campaigner Dave Sweeney said the spill was just another chapter in the long-running saga of environmental failings at Ranger.

He said a poorly engineered dam collapsed in December spilling six million litres of radioactive water into creeks which flow into Kakadu, about 300km south east of Darwin.

There were also claims last year that the mine's tailings dam was leaking 100,000 litres of radioactive water into the earth below Kakadu every day.

"The difference with this latest leak is that there is hard scientific data that shows that there is impact in Kakadu," Mr Sweeney told AAP.

"The water above the mine site does not have the contamination and the water below does."

He said the negative impacts on Kakadu, caused by the uranium mine, were growing daily.

"The existing operation is ageing and underperforming," he said.

"We have to reduce the impacts of the existing operation.

"Ranger should not be permitted to expand.

"If they expand they will be putting short term commercial considerations ahead of the Australian environment."

Mr Sweeney said regulatory regimes were not getting to the source of the problem.

He said ERA was fined on two occasions in 2004 for breeches of occupational health and safety, when an operator mistakenly linked industrial contaminated water from the mine into the water supply.

"Workers showered in it, people drank it, local Aboriginal people drank it in Jabiru and 20-plus people presented with headaches, skin rashes and stomach irritations."

According to Fairfax, 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.