Last week, Treasurer Wayne Swan revealed that the minerals resource rent tax (MRRT) had raised just $126 million in its first six months against a full-year forecast of $2 billion, blaming the shortfall on volatile commodity prices.

The Greens have labelled the tax a ''dud'' and on Monday, introduced a bill into the lower house to fix a loophole in the tax, that they say would raise $2.2 billion over the forward estimates.

''Gina Rinehart is popping the Moet,'' Greens MP Adam Bandt said as he introduced the bill.

''This tax was born from backroom negotiations, where the country's richest miners met with those supposedly representing the Australian public.''

The Greens bill – which would need support from Labor in order to pass – would mean that the government did not have to give money back to the miners every time the states raised mining royalties.

On Monday, before the introduction of the bill, Dr Emerson said the MRRT was designed to be flexible and there were no plans to change its form.

''The Coalition complains when it's not collecting revenue, they complain when it is collecting revenue,'' he told reporters in Canberra on Monday.

But during question time, Prime Minister Julia Gillard and Mr Swan did not rule out changes to the mining tax, when pressed directly on the issue by the Coalition.

Ms Gillard said the government was critical of the reckless approach of state governments to increase royalties, which are refunded to mining companies who pay them through the MRRT.

''Interestingly enough the opposition that has always criticised an efficient profits-based tax in minerals has gone tick, tick, tick to Liberal royalty increases around the country,'' she said.

Ms Gillard said the issue had been taken to the GST distribution review panel, which found the situation was both unsustainable and desirable, and was now being discussed with state treasurers.

The Opposition, which plans to scrap the tax in government, said Ms Gillard and Mr Swan should take the blame for negotiating the 30 per cent tax with the three big mining companies, BHP Billiton, Rio Tinto and Xstrata.

''The Coalition has said for some time that Labor's MRRT was a fiscal train wreck in the making,'' shadow assistant treasurer Mathias Cormann said in a statement.

In question time, Opposition Leader Tony Abbott asked Ms Gillard if she still had confidence in her Treasurer.

"The answer is, 'of course'," The Prime Minister replied.

MRRT revenue is supposed to fund a number of initiatives, including the tax concessions from an increase in the superannuation guarantee from nine per cent to 12 per cent and the removal of the 15 per cent super contributions tax for low-income earners.

Superannuation Minister Bill Shorten said the measures would be funded from consolidated revenue.

''We'll pay for this tax that we've cut and the Liberals want to reintroduce,'' he told ABC radio, adding 3.6 million Australians would be paying more under a Coalition government.

''Not even the Sheriff of Nottingham could have dreamed that up.''

Mr Bandt told Parliament that if the tax was not fixed before the budget, Australians could expect ''more Labor cuts, more hits to our science and research sector and more delays to the much needed public school spending boost''.

The Greens told reporters last week that they thought Mr Swan had opened the door to Labor supporting the bill.

''We do welcome the fact that at his press conference, the Treasurer has said that he is prepared to look at other factors other than commodity prices,'' Senator Milne said on Friday.

Debate is due to resume on the bill in the next sitting period in March.

With AAP