Serious doubts are emerging about whether the Gillard Government's mining tax will collect the billions of dollars in revenue predicted from the nation's three biggest resource companies.

Industry sources say miners' ability to claim depreciation of existing multi-billion-dollar assets against their mineral resources rent tax liability will significantly reduce the expected tax take from BHP Billiton, Rio Tinto and Xstrata.

By comparison, mid-cap miners who had relatively small assets as of May 2 last year - the point at which asset worth is effectively frozen for purposes of depreciation - will likely feel the full effect of the MRRT.

It has prompted concerns within the industry that the MRRT will affect miners unequally and the biggest companies will use the depreciation clause in the draft MRRT legislation to vastly limit their exposure to the tax.

The Government insists the tax, which like the carbon tax is scheduled to start on July 1 next year, will operate fairly across the industry and that the biggest miners will pay the lion's share of the predicted $11.1 billion in the first three years.

Treasury is predicting the MRRT will raise $3.7 billion in 2012-13, $4 billion the following year and $3.4 billion in 2014-15.

A spokesman for Treasurer Wayne Swan said the Government stood by the figures.

"The Government has full confidence in all Budget forecasts, including revenue forecasts," he said.

But one tax expert, who did not wish to be named, said there was a risk the Treasury's revenue forecasts would prove too optimistic because of the concession handed to the big three.

"The upfront concession is certainly worth a lot to BHP, to Rio, to Xstrata, and that's the way the tax is designed," they said.

This week, Fortescue Metals boss Andrew Forrest complained the big three had won a huge tax windfall with the deal struck in July last year.

"They will pay a lot less profits, a lot less tax per dollar profit than the Australian developer will, and I think that's terribly important," he said. "If they pay one cent more as a multinational and we pay $10 more, do you think that's fair?"