The federal opposition is demanding Treasurer Wayne Swan explain a potential $4 billion black hole in its mining tax that is due to start in July.
Joe Hockey and Senator Mathias Cormann, the opposition's treasury and assistant treasury spokesmen, say the cost of all the promises the government has attached to its minerals resource rent tax far exceeds the revenue it is expected to raise.
"The government must immediately release all modelling, assumptions and outcomes that underpin their mining tax," they said in a joint statement.
"The government cannot expect the Australian parliament to make fully informed decisions about the mining tax without the full details of this $10.6 billion tax grab."
The 30 per cent MRRT has been passed by the House of Representatives, but still needs approval from the Senate before it passes into law.
Revenue from the tax will help fund a cut in the business tax rate, small business tax breaks, infrastructure spending and increases in superannuation.
Mr Hockey and Senator Cormann specifically want to know whether the cost of the commitment to credit all state and territory royalties against any MRRT liability has been factored in. If not, it will leave an unfunded hole in the budget.
Since the MRRT was announced in mid-2010, the states of Western Australia, NSW, South Australia and Tasmania have said they intend to increase their royalties on either iron ore or coal, meaning the government will have a bigger task crediting miners with royalties paid.
Mr Hockey and Senator Cormann also want to know why the government has not provided the mining tax revenue assumptions, including crucial details such as commodity price and production volume assumptions, and the full costings of all associated measures.
"The coalition has been calling for the release of all details for the last 18 months," they said.
"The Australian government is treating all Australians with contempt by not releasing this information."
