Why won't Wayne Swan and Julia Gillard tell the truth about how much revenue they gave up to the big resources companies to stop their campaign against the proposed "super profits" mining tax?

They want to have it both ways. The Prime Minister and Treasurer want voters who are worried about Labor's policy bungles and backflips to believe that this time they drove a hard bargain.

At the same time they're winking at the big end of town: you were right, the tax was unworkable, an unfair dud, so we've junked it for a less onerous ''mineral resources rent tax'' designed by the three biggest miners themselves.

The government's dishonesty and lack of transparency goes back to the unveiling of the tax. When first announced on May 2, the Treasury forecast it would raise $12 billion in its first two years.

Controversy immediately erupted over whether resources companies had been paying ''fair'' taxes and royalties. Swan rushed out work claiming miners paid only 17 per cent tax. When swiftly discredited, he dribbled out implausible figures on mining earnings, taxes and royalties.


Yet Treasury's estimate of revenue for the same two years was $10.5 billion, only $1.5 billion lower. It barely seemed possible. Gillard and Swan had satisfied the miners while collecting almost all the tax.

Swan refused to tell us the Treasury's assumptions about prices and volumes underpinning the estimates, nor have we been told what revisions have been made to support the estimates.


If Swan wants to maintain a shred of financial integrity as Treasurer, he must immediately publish the major assumptions behind the estimates for the original tax and how they were changed for the revised one. He should release these in a form enabling us to compare both sets on the basis of common assumptions. That is the only way we will know how high a price the Government paid to get the mining industry off its back.

The Treasurer's lack of candour on this issue extends to the long-term budget impact of more generous rules on valuing mines and depreciating capital under the minerals resources rent tax. These rules were redesigned by the three companies set to pay most of the new tax - BHP Billiton, Rio Tinto and Xstrata.

Conveniently for Swan, these concessions mostly affect revenue beyond the forward estimates so he doesn't have to admit their real cost. The government has refused to provide the Senate, media or public with any long-term projections for either variation of the tax, only for the first two years.

What a contrast to May last year, when a Coalition proposal to retain the private health insurance rebate and offset its cost by increasing tobacco excise prompted the government to leak misleading 10-year projections. Or to last year's negotiation over emissions trading, when it demanded Coalition amendments be costed over 10 years (which they were). Apparently only the opposition should be accountable for long-term financial responsibility.

We do, however, have private estimates of the concessions. JBWere Goldman Sachs, for instance, estimates the net budget cost at $35 billion, implying the modified tax will raise barely half the revenue over 10 years.

The Coalition believes the new minerals resources rent tax is no more justified than the old resource super profits tax. But beyond the rights and wrongs of higher taxes on mining, how can Australians trust a government that relies on secrecy and misleading information to justify its policies?

Malcolm Turnbull is the Liberal MP for Wentworth.