The mining sector has added its booming voice to the chorus of discontent over proposed adjustments for more federal money.

Among the suggestions in the recent Commission of Audit report is a call to cut a diesel fuel subsidy for farmers and miners.

The subsidy removes road taxes for fuel used off-road, and is worth bllions of dollars to mining companies nationwide.

ABC has published details from a confidential communiqué, which warns of the potential impact of the cost-cutting idea.

“We've run the numbers on any substantial change to the rebate and the impact would be profound.

“Most likely far greater than any MRRT [Minerals Resource Rent Tax] and probably a little less than the first mining tax,” it read.

The leaked letter was allegedly written by a key mining industry CEO.

It explains that after labour costs, fuel is the next biggest expense for miners.

“With so many projects in their infancy or in early stages of developments in three states - such changes would alter the landscape for investment - and no doubt spook financiers,” the message said.

Many commentators say that the subsidy should be retained for farmers, but that mining companies should have to pay for their own fuel given their much more significant profit margins, pollution emissions and fuel usage.

Rio Tinto iron ore boss Andrew Harding said at a business lunch in Perth last week that the possibility of changes to large taxes “really does startle an organisation”.

“Australia’s not the only place you can mine iron ore,” he said.

“There’s an awful lot of high grade iron ore sitting in Africa, and for a whole lot of instability reasons it hasn’t been mined to date.”