An investigation commissioned by the Ranger mine in Kakadu has found a worn out rubber tank lining was the cause of a major leak of radioactive acidic slurry almost four months ago.

Leach Tank 1 collapsed on December 7 last year, releasing about one million cubic litres at the mine site, located within the boundaries of Kakadu National Park in the Northern Territory.

The rubber lining inside the tank, which protects it from corrosion, had been damaged as a result of wear from a partially failed baffle, or flow director, the investigation found.

"The damaged rubber lining allowed the acidic slurry mixture to come into contact with the tank's steel wall, which subsequently corroded and ultimately led to the failure of the tank," mine operator Energy Resources Australia (ERA) said in a statement on Thursday.

The tank was the only one of seven which had been modified to process laterite ore, and the change in ore type involved modifications to Leach Tank 1 that included the addition of a higher powered agitator, to which the partial failure of the baffle has been attributed.

Inspections are underway of the other six leach tanks, ERA says, and metal fatigue has been detected in the baffle supports in Tank 6.

Although its baffles were operative, ERA will redesign and replace the baffle supports in all of the leach tanks before they are returned to service, CEO Andrea Sutton said in a statement.

Seven critical actions have been identified which ERA must address before processing resumes at the site, relating to tank inspections and thickness testing for all leach tanks and other processing plant tanks.

A further 28 actions - all accepted by ERA - were recommended to be undertaken on specific assets or systems before they are returned to service.

The investigation commissioned by ERA is separate from the joint investigation being undertaken by a government-appointed taskforce charged with overseeing the regulatory response to the leak.

ERA maintains that the spillage was fully contained on site, with no impact on Kakadu National Park.

The Gundjeihmi Corporation, which represents traditional owners the Mirarr people, is awaiting the regulators' response, said CEO Justin O'Brien.

"It would be premature to consider the statement as conclusive," he told AAP.

The Environment Centre NT has called on regulators to close the mine to prevent another major incident.

"It's alarming that ERA's own appointed investigators found that alongside the failed leach tank, Ranger mine had another 35 pieces of critical plant infrastructure and equipment on the brink of failure that were unsafe to be in operation," said Lauren Mellor, spokeswoman for the Environment Centre NT.

"It's clear that there was a complete absence of routine periodic equipment inspections, and that existing mine regulations are insufficient to prevent this type of accident from happening again."

She said Ranger's plant infrastructure and equipment is well past its intended design life, and if allowed to continue operations will increasingly be putting workers, local communities and the environment at risk.