Independent MP Rob Oakeshott feels the Gillard government partly duped him about the design of its mining tax.

The MP, who backed Labor's minerals resource rent tax (MRRT) in the parliament, says the government told him that hiked state government royalties would not be offset against the MRRT.

The 30 per cent tax on the super profits on coal and iron ore miners so far has failed to meet government revenue estimates.

In the six months to the end of December the MRRT raised just $126 million against a full year forecast of $2 billion.

When asked whether he felt duped by the government, Mr Oakeshott told ABC Radio on Tuesday: "Partly ... yes a bit duped."

The MP said he had a letter from the government indicating its willingness to implement a recommendation from the Henry tax review to replace state mining royalties with a rent resource tax.

"They recognised royalties are unsustainable and a bad tax for Australia and a better way forward is with a national resource rent tax."

The government said it would look for ways to penalise and reduce the opportunity for royalties to be increased and offset against MRRT liabilities, Mr Oakeshott said.

The MP has seconded an Australian Greens bill that closes that loophole.

Assistant Treasurer David Bradbury ruled out any change to the MRRT, saying the government will continue to monitor the revenue receipts "as and when they come in".

"This is a very volatile tax, resource rent taxes always are," he told reporters in Canberra.

Mr Bradbury said the question of royalties would be dealt with the states in the GST redistribution review.

The government also would address any budgetary implications in the May budget.

Independent MP Tony Windsor said the issue between royalties and the MRRT rent must be addressed.

"If there is structural issues, let's fix them," he told reporters.

"If it's only profit issues, well leave it alone."

Mr Windsor said he had not received any correspondence from Prime Minister Julia Gillard about the MRRT.