Taxpayers are exposed to a $3.2-billion black hole in funding for the future environmental clean-up of the state's coal mines, an internal Queensland Government report has warned.

Key points: The report found there was a shortfall in funds to rehabilitate several mine sites

The report found there was a shortfall in funds to rehabilitate several mine sites It found inaccurate, incoherent financial assurance calculations by some companies

It found inaccurate, incoherent financial assurance calculations by some companies The department said the report has not been "ratified by senior management"

The report titled Financial Assurance for Queensland Coal Mines, obtained by the ABC, found that 90 per cent of the financial assurance (FA) put forward by the industry was "incorrect" and "too low".

But the department responsible for the report has told the ABC its calculations have not yet been ratified and are not "reliable".

FA is provided to state governments by mining companies in the form of bank guarantees or cash, and is held as a bond for the environmental remediation and rehabilitation of mine sites.

The FA report was finished in January this year, and it examined a sample of 15 Queensland coal mines.

The mines selected represent a quarter of all coal mines in Queensland.

Of the 15 in the sample, some were closed, others were in "care and maintenance", while the remainder were likely to cease production in the next five years.

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The report said financial assurance calculations by the companies were "generally incomplete and inaccurate, and sometimes incoherent", with "significant errors" in the calculation of FA for 13 of the 15 mines.

Overall, it found there was a shortfall of $840 million to "complete rehabilitation" of the 15 sites.

That figure was then extrapolated statewide, finding the "amount of FA held is currently insufficient to complete rehabilitation of coal mines in Queensland".

$3.2 billion is 'the tip of the iceberg'

Queensland's Isaac Plains coal mine was shut down in 2014. ( ABC News )

It estimated an additional $3.247 billion was "required to complete rehabilitation of coal mining's disturbance footprint in Queensland".

"This footprint is estimated to have now reached 190,000 hectares," the report said.

Rick Humphries of the environmental group Lock the Gate Alliance said the $3.2 billion figures was just for the state's coal mines.

"[It] doesn't include gold, copper, lead, zinc or bauxite," he said.

"So it's just the tip of the iceberg in terms of a huge mining legacy faced by the taxpayer in Queensland."

Gavin Mudd, head of Environmental Engineering at Monash University, said the report underlined the possible shortfalls in environmental bonds held by governments around Australia.

"If Queensland has undervalued their rehabilitation costs and liabilities by $3 billion or so, then the rest of the country would certainly be similar," Dr Mudd said.

"As taxpayers we shouldn't be having to cough up money to pay for something a mining company should've already paid for."

Queensland collecting 'far too much' financial assurance

Norwich Park mine in central Queensland was shut down in 2012. ( ABC News )

But Michael Roche of the Queensland Resources Council said there was no shortfall in what the state was holding in financial assurance from mining companies.

"We think the state is collecting far too much compared to the risk the state is actually carrying site by site," he said.

"There are a lot of big global companies paying billions of dollars that really could be working to fund rehabilitation and really fund jobs, but at the moment are tying up capital."

An officer from the Queensland Environment Department was responsible for writing the internal report. That officer has now left the department.

In a statement to the ABC, the department said the "methodology and calculations found in this document have not yet been ratified by senior management".

The department said the "figure of a $3.2 billion shortfall was obtained by extrapolation" and that the 15 mine sites sampled represented a potential shortfall of only $30 million, not $840 million.

Queensland Environment Minister Steven Miles said the Government held nearly $7 billion in financial assurance for mining, gas and petroleum activities, up from $1.45 billion in 2008.

The New South Wales Audit Office is drawing up terms of reference for a review into mine rehabilitation deposits in that state, in response to concerns that a decline in the coal sector could leave taxpayers liable for clean-up costs.

Mr Humphries is a former advisor to Rio Tinto on land use. He argues that with mining boom over, governments need to ensure taxpayers are put above the interest of company shareholders.

"When [companies] got permission to mine, part of the deal was you'll clean up the mess after you've finished making the profits," he said.

"Now, having made their money, they are trying to cut and run, and that's not acceptable."

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