The Federal Opposition has dismissed an open letter from 20 economists who are backing the Government's new mining tax.

The 40 per cent tax on mining profits is dominating parliamentary debate, with the Government and Opposition continuing to trade blows over whether the industry is paying enough tax.

A group of prominent economists backs the tax, saying claims that it will push up the cost of living and slow growth in the sector are without substance.

But Opposition resources spokesman Ian Macfarlane has brushed aside the group's support for the tax.

"I think economists are put here to make weather forecasters look good," he said.

"I was at a luncheon with [Reserve Bank governor] Glenn Stevens in Toowoomba only a month ago where his opening line was, 'Yes I know everything I predicted 12 months ago didn't happen'."

Opposition finance spokesman Andrew Robb says the economists are debating the wrong point.

"This debate has never been about the design of the tax," he said.

"The debate is all about the size of the tax grab. The economists are attempting to shift the focus."

Meanwhile, Treasurer Wayne Swan has seized on comments by Opposition Leader Tony Abbott in which he suggested that the mining industry is paying too much tax.

Speaking on 2GB radio Mr Abbott said: "Any fair-minded analysis of the evidence would suggest that mining companies, when you throw in corporate taxes and royalties, are paying more than their fare share of tax.

"Yet because it suits the political purposes of this Government they're being blackguarded up hill and down dale."

Mr Swan says Mr Abbott's comments are "extraordinary".

"Not even the miners believe they are paying too much tax," he said.

"That just shows how far Mr Abbott is out there, with people like [mining magnate Clive Palmer] arguing in the commercial interest of some miners and not arguing in the national interest."

When Mr Macfarlane was asked by reporters if the industry was paying too much tax he replied: "I think they're paying a fair amount of tax and I think that if there needs to be a change to that there needs to be a long and detailed consultation initially between the states and the companies involved."