The head of Treasury has hit back at the Coalition's claims that the federal budget forecasts were fudged to suit the Government.

Coalition MPs have accused Treasury of being politicised and implied the budget forecasts are deliberately optimistic.

The Coalition argued that the Australian public would get the real numbers, untainted by the hand of the Government, in the Pre-Election Fiscal Outlook (PEFO).

But Treasury Secretary Martin Parkinson has hit back at those claims, saying had the PEFO been released on budget day it would have contained the same numbers as the budget.

Dr Parkinson used a speech in Sydney entitled Budgeting in Challenging Times to defend his department's forecasting record.

Mr Parkinson admitted that "while our forecasts in the real economy have held up reasonably well, the same can't be said of our price forecasts".

The Government has blamed an unexpected slump in the growth of nominal GDP - or the price of goods produced - for a gaping revenue shortfall of $60 billion over the forward estimates and its failure to deliver a promised surplus this financial year.

Dr Parkinson says the Government makes its own decisions based on the estimates Treasury provides.

He says the department is not swayed by its political masters.

"Let me be very clear: Treasury does not provide the Government with a range of numbers; Treasury provides its best professional estimate to the Government," he said.

"It is up to the government of the day - and this applies back through history - to do what it wishes with those forecasts."

Dr Parkinson says it has been a "tumultuous period" for Australia's economy.

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"In these circumstances, we in Treasury have struggled to keep pace, and the result has been large revisions to our economic and revenue forecasts," he said.

"The result has been to undertake a comprehensive review of forecasts overseen by an independent panel."

The Opposition has poured scorn on treasury's figures, saying they are "Wayne Swan's numbers".

The morning after last week's budget was handed down, the Opposition treasury spokesman Joe Hockey said he did not believe the budget forecasts.

"Well I don't believe they're Treasury's numbers," he said.

"I think they're Wayne Swan's numbers and that's a starting point because his name is on the front of the document."

But Finance Minister Penny Wong says Dr Parkinson has destroyed the credibility of the Coalition's criticism.

"Today what we've seen is the Secretary of the Treasury [completely] destroy another of the excuses that Joe Hockey and Tony Abbott keep using to keep Australians in the dark," she said.

A spokesman for Mr Hockey says he maintains the numbers in the budget are Mr Swan's.