Rio Tinto's relationship with an important indigenous group in Kakadu National Park has taken ''two steps backwards'', after a safety breach at the Ranger uranium mine in recent days.

The Rio subsidiary that operates Ranger, ERA, has confirmed that one of the vehicles used at the mine was taken out of controlled areas, sparking contamination fears from the Mirrar people who live nearby.

Police are investigating the incident, which took place without the consent of ERA management in the early hours of Sunday morning, and which some believe could amount to a breach of the company's authorisation to mine.

Like all uranium mines, Ranger operates under strict conditions in a bid to ensure that dangerous levels of uranium do not contaminate the nearby area.

ERA said the car - which was supposed to remain inside the mine at all times - had been checked and was ''free of contamination'', and that the matter had been referred to police.

But Justin O'Brien, who represents the Mirrar people, said it had caused great concern among the local community.

''We think it is very serious that you could take potentially contaminated material from an operational mine site, avoid all scrutiny, leave the mine site with it and then be found down the highway,'' he said. ''There needs to be a broader inquiry into how on earth this could happen in the first place.''

ERA's relationship with the Mirrar people is crucial to its survival, given it has agreed not to restart mining at Ranger without approval from the group. Ranger ceased operating as an open cut mine last year, and its only future lies in winning approval to become an underground mine in coming years.

The highly prospective and nearby Jabiluka uranium deposit will also not be mined until the Mirrar give their full support, something that appears highly unlikely to occur any time soon.

The Mirrar people had recently spoken of their improving relationship with ERA and Rio Tinto, which was given a boost in September when ERA installed a $220 million water purification system on site.

But Mr O'Brien said incidents like the one on the weekend did not help Rio's chances of winning further approvals. ''It can only stress the relationship, it can only challenge the relationship and the test is how the company responds to this in terms of its management of the investigation and its response with the contractor,'' he said.

''It's frustrating. One feels as if you take a step forward and then you are dragged two steps back.''

Australian Conservation Foundation spokesman Dave Sweeney said it was not the first time breaches had occurred at Ranger, and it was now time for Rio Tinto to ''reconsider the project''.