THE Gillard government's mining tax has been thrown into doubt with the Greens resolving to oppose the billions of dollars in company tax cuts the minerals impost was to fund.

The Greens leader, Bob Brown, will tell a welfare conference today the 30 per cent company tax rate in Australia is low enough and the mining tax revenue slated to fund a 1 percentage point reduction should be used to fund a national dental scheme or bolster welfare payments.

The government will introduce legislation for the minerals resource rent tax after July 1, when the Greens will hold the balance of power in the Senate.

The Coalition opposes the mining tax, meaning the Greens' support is crucial.

After growing increasingly critical of the concessions the government has made to appease Rio Tinto and BHP, Senator Brown has drawn the line.