West Australian mining magnate Andrew Forrest has savaged Treasurer Wayne Swan over Labor's mining tax, saying it won't deliver the $10 billion expected by the federal government.

Mr Forrest also ruled out entering politics but said he wasn't surprised his mining contemporary Clive Palmer wanted to challenge Mr Swan in his federal seat of Lilley at the next election.

"I'd be surprised if Mr Swan contests that seat because I think a three-legged dog could beat him to it," Mr Forrest told the National Press Club in Canberra on Wednesday.

The Fortescue Metals Group founder said a lot of successful people were approached to enter politics but he was focused on his philanthropic endeavours.

Mr Forrest is a vocal critic of Labor's Minerals Resources Rent Tax (MRRT), which comes into effect on July 1, but his iron ore company expects to pay little or none of it.

"I would say we will pay between zero and $50 million of MRRT some time in the next five years. I am not sure when it is and it will depend on iron ore prices," he said.

The Fortescue non-executive chairman also said the tax won't raise the estimated $10 billion in funds the government wants to use to pay for superannuation increase, tax breaks for small businesses and infrastructure.

"He (Mr Swan) ran out and spent all the money while knowing it was an apparition ... it was a mirage," Mr Forrest said.

He said Fortescue was now paying its share of corporate tax, after previously absorbing losses before its Pilbara iron ore projects went into production.

Mr Forrest also said the ongoing instability around the minority Labor government was shaking confidence in the business community.

"That is dangerous for the economy and it should be resolved as soon as possible," he said.

Mr Swan's office said Mr Forrest was talking down the economy.

"Mr Forrest - just like Clive Palmer and Tony Abbott - doesn't want Australians to share in the benefits of the resources that we all own," a spokesman said.

"They can stand up for vested interests and their mining profits - the government will keep standing up for working Australians."

Fortescue plans to mount a High Court challenge against the MRRT on constitutional grounds within weeks.

AAP