The dollar figure of the security bond for the NT's McArthur River Mine, 40 kilometres from the community of Borroloola, should be made public, the Environmental Defenders Office (EDO) says.

Key points: NT EDO calls for dollar figure of McArthur River Mine's security bond to be made public

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The EDO's principal lawyer, David Morris, said he had been trying to find out the details of the interim increase to the security bond, agreed to last year by the NT Government and McArthur River Mining (MRM), one of the world's biggest producers of bulk zinc concentrate.

Through a Freedom of Information request to the Department of Mines and Energy, the EDO obtained correspondence between the department and MRM about negotiations over the increase.

Mr Morris said the letters shed light on the talks, but that the dollar figure of the bond was redacted from the documents on the grounds the figures constituted "commercial and business" information and were therefore exempt from the Information Act.

Last year the NT Government demanded an interim increase in the bond because of the amount of potentially acid-forming waste rock at the site.

Indigenous residents raised concerns about environmental impacts from the mine after elevated lead levels were found in fish on the mine site.

A contamination incident last year also resulted in cattle having to be destroyed.

"My opinion is that there's a real public interest in the community being able to understand and know for sure what sum is held by the Northern Territory Government as security to rehabilitate this site and my opinion again is that the public interest outweighs any consideration of the commercial and business interests of MRM in this case," Mr Morris said.

In a statement, the NT Department of Mines and Energy defended keeping security bond figures confidential and said the dollar value of the security bond for MRM was "very commercially sensitive information".

"These are very large amounts of money; releasing that information publicly would have significant implications for the company, potentially placing them at a competitive disadvantage," the statement said.

The mine's operator said its security bond had been calculated in line with the NT Government's established process and formula for determining security bonds for mining operations.

Increased bond grounds for public release: EDO

Last year the Northern Territory Government and MRM were in talks about an interim increase to the security bond for rehabilitating the site.

In August, the Government said the bond held was deficient.

"The major issue is the amount of reactive rock coming out of the mine, which was on a scale no-one anticipated, and the result is that the final remediation costs have increased," a spokesman for Mines Minister Dave Tollner said at the time.

In correspondence from October last year released through Freedom of Information to the EDO this week, MRM wrote that it was willing to agree to an additional security on the condition the department of mines approve a mining management plan.

In its response later that month, the Department of Mines, appeared to rejected that condition and warned the mine that approvals would be revoked if the company did not agree to the new figure.

The amount under discussion was redacted from the documents obtained by the EDO, but Mr Morris said the figure and how it was calculated should be made public.

"The Government should be telling us what is the Territory's exposure to rehabilitation on this site," he said.

Mr Morris said a report by the mine's independent monitor released last year showed some of the costs of closing McArthur River Mine remained unknown.

The report for the mine's operations in 2014 found that there was potential that "perpetual management of the site may be required", but the report said the company did not appear to have allowed for that cost.

"They seem to say that management of this site is probably going to be required forever, and so how you factor that into a closure cost and how you factor that in then to a rehabilitation security levy, which is supposed to cover 100 per cent of closure costs, is a question that remains unanswered by the mining company and the Northern Territory Government," Mr Morris said.

The Independent Monitor's report said the company was working on a plan to manage waste rock at the mine that was expected to provide more clarity on future management costs.

In a statement, MRM said its closure planning process considered economic, social and environmental factors to provide for a balanced and sustainable outcome after mining.