The Northern Territory government's decision not to prosecute a uranium miner for a major contamination spill near Kakadu National Park is a kick in the face for the public, an environmental lawyer says.

The Department of Mines and Energy on Friday said it was not in the public interest to press charges against Rio Tinto subsidiary Energy Resources of Australia (ERA) for breaching the Mining Management Act when up to one million litres of contaminated acidic slurry spilled from a collapsed leach tank in December 2013.

Department CEO Ron Kelly said there was doubt about whether ERA directly caused the tank to collapse by failing to monitor it or ensure it was strong enough to contain the slurry.

The Supervising Scientist's investigation found that the spill was contained on the mine site and did not impact the surrounding national park, or the health of workers or local residents.

ERA has welcomed the decision.

But David Morris, principal lawyer with the NT's Environmental Defenders Office, said regardless of why the spill occurred, ERA was responsible.

"If this is a government trying to build trust with the public and build its credibility as an environmental regulator, this is another kick in the face to a public that I think is really crying out for some responsible regulation of the mining industry," he said.

ERA submitted quarterly audits over the past year and was making significant progress in implementing a safety program to reduce the risk of a similar incident happening again, Mr Kelly said.

The company suspended its operations for six months following the leak, which resulted in a net loss of $70 million compared with the same period the previous year.

Justin O'Brien, CEO of the Gundjeihmi Corporation which represents the Mirarr traditional owners of the site, expressed concern about the site's prospects for rehabilitation.