The Prime Minister and Treasurer have dismissed comments by Fortescue Metals Group chairman Andrew Forrest that he had brokered an in-principle agreement with Kevin Rudd on a new mining tax.

The deal with Mr Rudd was made just before the former prime minister was rolled from the top job in June 2010 and was due to be unveiled as early as the day after the leadership coup, according to reports.

The proposal would have seen Fortescue's mining tax liability effectively go from 40 per cent to 20 per cent thanks to generous infrastructure write-offs.

Mr Forrest told ABC's PM the new agreement would have resulted in a injection into infrastructure.

He alleges that deal was being undermined by Treasurer Wayne Swan and Prime Minister Julia Gillard who were working on a deal with BHP and Rio Tinto before they toppled Mr Rudd.

He says Mr Swan knew of Mr Rudd's talks with Fortescue Metals because the Treasurer's chief of staff was involved and kept abreast of the discussions.

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'Old nonsense'

Mr Forrest was a vocal opponent of the Resource Super Profits Tax, which was the predecessor to the current mining tax, the Minerals Resource Rent Tax (MRRT).

Prime Minister Julia Gillard told ABC Local Radio there was no truth to Mr Forrest's claims.

"In today's newspapers we are just seeing more rehashed old nonsense," Ms Gillard said.

"The main fact here - the thing that really matters - is that after I became Prime Minister I sat down with the biggest miners in the country and worked through to deliver the Minerals Resource Rent Tax."

Mr Swan says the West Australian mining magnate is notorious for making extraordinary claims.

"There's all sorts of claim and counter claim, it just gets weirder," Mr Swan said.

"You see, you've had Mr [Clive] Palmer out there with all of his conspiracy theories, now we've got Mr Forrest making all sorts of claims. What happened here is that Julia Gillard and I got this done."

But Mr Forrest says Mr Swan is wrong.

"To try and write it off as a conspiracy theory... when there's total government evidence for every word I've said is to really insult the average Australian," he said.

During a speech in Sydney yesterday, Mr Forrest attacked the MRRT, which he said would "take from the poor and give to the rich".

He said the largest mining companies were allowed to negotiate the tax with the Federal Government in secret while smaller companies were shut out of the process.

"It gives a huge tax concession for being big, which of course if you're small you don't get," Mr Forrest said.

"And I just don't see any precedent for that, where you apply a full tax to a small part of the industry, yet say if you're big you get let off."