The three big miners know more about the Gillard government's proposed new resources tax than the Australian people, the opposition claims.
The government continues to defy a Senate order to release the figures and assumptions that back up its projected revenue of $10 billion from the minerals resource rent tax.
It negotiated the make-up of the 30 per cent tax with the nation's three biggest miners - BHP Billiton, Rio Tinto and Xstrata - but upset small and mid-tier miners in the process.
Treasurer Wayne Swan on Thursday argued it would be "extremely damaging" to the three companies and to the national interest if commercial-in-confidence information were published.
"Treasury never has ... published the detailed information about individual commodities," he told ABC Radio.
"(It) is very market sensitive and it is simply absurd to suggest that sort of information should be put into the public arena."
Opposition assistant treasury spokesman Mathias Cormann questioned what the government was hiding in the numbers.
Successive West Australian state governments released the sorts of assumptions the Senate was seeking as a matter of course, he said.
"The three biggest miners are allowed to know what assumptions are used by the government but nobody else is," Senator Cormann told AAP.
"Not the other companies, not the parliament and not the people."
That was "entirely inappropriate and completely inconsistent" with the government promises of a more transparent and open government."
Independent senator Nick Xenophon, who supported the coalition move in the Senate last week, said there had been a lack of transparency in the debate about the tax and too little information had been made available.
"Poor policy may well have come out as a result of that," he told ABC Radio.
Senator Xenophon was also worried about the three big miners having a commercial advantage over their smaller rivals.
"The issue of the future of Australia's mining industry will be driven by the new and emerging miners in the long term," he said.
"Having a deal with just the big three ... is unfair from a commercial point of view because they're in the loop where all the (other) miners are out of the loop."
Mr Swan said a government-appointed panel, chaired by leading businessman Don Argus, was refining the design of the tax due to operate from July 2012.
The review will enter deliberations with the mining sector in Perth later on Thursday.
