The McArthur River Mine has revealed that it accidentally dumped 14,000 tonnes of potentially acid-forming material at the wrong site, where it combusted, but the Northern Territory Government has said it is satisfied with how the matter was handled despite the lack of existence of a written report.

On Thursday, an independent monitor's report on the mine was released, detailing how the long-term management of potentially combustible toxic waste rock continued to be an issue at the site.

On the same day, The Guardian reported that Glencore had quietly included details of another waste rock incident in its mammoth draft environmental impact statement (EIS), which is currently being considered by the NT and Federal Government as it seeks to extend the operating life of the mine by another 30 years.

During the incident, 63 truckloads — or 14,000 tonnes — of potentially acid-forming rocks were mistaken for more benign material and "incorrectly" dumped in the mine's southern waste facility, which is not designed to hold that type of waste.

The material then combusted, sending sulphur dioxide into the air.

The acid-forming rock waste is supposed to be taken to the northern waste facility, where it is covered with clay and rock. This aims to protect those rocks, which exist naturally and are harmless when located deep below the groundwater table away from oxygen.

But the clay and rock does not always succeed in protecting the rock.

Exposure to water and air can cause them to become acidic and catch fire, similar to what happened during a 2013 plume at the dump, which emitted toxic smoke from combusting rocks for more than a year.

The dumping mistake was discovered after plumes of emissions were again spotted rising from the mine site in August 2016, shortly before the NT election.

The then-CLP Government ordered restrictions on mining at the site and said officers from the Department of Resources and Primary Industry were investigating.

A paragraph in the company's draft EIS said the potentially acid-forming material made up about 0.24 per cent of the total mass of waste in the southern facility, but it also detailed special measures taken before the 2015 wet season in an effort to direct run-off from the "non-benign" material to the contaminated water collection system.

The McArthur River Mine tailings dam. ( ABC News: Jane Bardon )

NT Government satisfied with how incident was handled

The incident was investigated thoroughly, said Sam Strohmayr, general manager at the mine.

"We've looked at how that happened, as you do in all incidents, and we've come up with a bunch of different ways to make sure that it doesn't happen in the future," he said.

"You've got to put that in perspective — that's a waste rock pile, the loads and material that went there by accident is about one quarter of one per cent of the total volume of that waste rock pile.

"We do extensive monitoring in that area, so the impacts of that, we haven't seen any at all.

"Like all of our incidents, we take them very seriously, irrespective of how minor or large; [it] was investigated properly, internally, so we come up with the controls to make sure those things don't happen in the future."

Acting Chief Minister Nicole Manison said the NT Government wanted to ensure that all mines met their environmental safety obligations.

"That's why it's important to have monitoring practices and these checks and balances in place, because if something is not done to best practice or something goes wrong, it's very important to make sure it doesn't happen again," she said.

The Department of Primary Industry and Resources was happy with the way the incorrect dumping was handled, spokesman Armando Padovan said.

"I think the environmental harm done is really limited to sulphur dioxide being limited to the atmosphere," he said, but was unable to detail how long that was happening before the error was discovered.

"There's no evidence of any damage to the river system; whilst the number of trucks was 63, the amount of material in the context of the mine is actually relatively small."

He said the Government was unable to publicly release further details because the matter was reported on verbally, with no written report.

"The way we normally deal with a lot of incidents, it depends what the matter is," he said.

"We were able to resolve this one fairly quickly and move on to the solution so there is no incident report."

When asked why the Government did not inform the public about the results of the investigation into the incident and the actions being taken in response, a spokeswoman for the Mines Minister said the mine had acted within its mine management plan "so there was no requirement to report anything. It [theplan] has now been updated".

And when asked whether it was acceptable practice for investigators from the Department of Primary Industry and Resources not to have written any kind of report about their findings and instead handled the matter verbally, she said the department requested that the mine complete a report on the incident, which they did within the time frame.

"It [The Department] then consulted an independent investigator to look into this matter. This quickly showed MRM acted within their MMP, so rather than spend unnecessary resources on a report that would not result in prosecution, the Department focused their energy on ensuring appropriate action was taken to remediate the situation," she said.