The company that owns the Ranger Uranium Mine and regulators are looking into how radioactive waste ended up in Winnellie after being transported through Kakadu National Park from the mine site in June.

The excavator at Ranger Mine used to dig uranium tailings was removed from the mine site loaded with 50kg of mixed material.

Under Energy Resources Australia’s mine rehabilitation plan, removing any toxic waste is considered a significant breach as it could present a deadly contamination risk to both people and the environment.

The excavator was taken to Winnellie, 257km from the site, for maintenance when the waste was discovered after disassembling the vehicle.

ERA said the compressed waste was in a steel-encased section of an excavator and not detected by radiation screening before leaving the site.

“Our investigations are ongoing, in consultation with stakeholders and regulators who have been briefed on the matter,” the company said, adding that the material was immediately returned to the mine.

“Due to its low levels of radiation and small scale, it presented no potential for dispersal or exposure to the community.”

Supervising Scientist Keith Taylor said he was confident there was no risk posed to the environment or any person.

He said scientists and ERA were working to assess the ‘clearance processes,’ including radiation screening.

Environment Centre chief executive Kirsty Howey told the NT News the incident was considered a “serious breach” despite no proof of harm being caused.

“It really raises questions about the integrity and the controls, including safety controls around the rehabilitation operation, and it’s extremely concerning that any material at all has been able to leave the site,” she said.

Early this year, ERA’s costs to rehabilitate the mine were estimated to reach $1.2 billion.

Rio Tinto holds 86 per cent ownership of ERA.

The mine, which operated for almost 40 years, produced more than 130,000 tonnes of uranium oxide and is currently under a five-year clean-up phase with strict requirements to be incorporated into the surrounding Kakadu National Park by 2026.