The Federal Government has questioned how shadow treasurer Joe Hockey can run the nation's finances when he refuses to accept figures provided by Treasury.

Mr Hockey has cast doubt over the independence of Treasury's pre-election budget update which is released 10 days into the official campaign.

He says the Coalition will not be relying on the figures to frame its policy costings because they cannot be trusted.

Mr Hockey has accused the Government of trying to influence Treasury by preparing its own economic statement to be released soon.

"Quite clearly by flagging an economic statement, the Government is trying to bully the public service into a set of numbers that clearly do not properly represent the state of the budget," Mr Hockey told AM.

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The Treasurer, Chris Bowen, says he rejects that assertion.

"It is an insult not to me but to the Treasury and the Department of Finance, and I am sick and tired of the Opposition trashing the reputation of Australia's public servants," Mr Bowen said.

Opposition Leader Tony Abbott has backed his treasury spokesman.

"Joe was making the point that figures from this Government have never turned out to be true; never, never, never starting from their very first budget."

The Government has described as "unprecedented" the Coalition's stance not to use Treasury's pre-election economic and fiscal outlook (PEFO) as the basis for its costings.

Mr Hockey said earlier this year "the only numbers than we can rely on are the PEFO numbers released by the heads of department during the course of the election campaign".

Mr Bowen says Mr Hockey's statements today represent a "remarkable" change of position.

"Clearly the Opposition is softening up the Australian people for not releasing their fully costed and funded policies and softening up the Australian people for not telling the true story about the state of their budget plans," Mr Bowen said.

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But Mr Hockey says the Coalition would base its policy costings on a range of different sources, and all the details would be made public "in the next few weeks".

The Coalition came under fire during the 2010 election campaign for using an accounting firm to cost its policies.

Mr Hockey said this time the Coalition would be using figures from the Parliamentary Budget Office, state Coalition governments and independent advisers.

"Our numbers will be more reliable than anything the Government publishes because every single number the Labor Party publishes in government is dead-set wrong," he said.

But Mr Bowen said: "If the Opposition has a different set of figures, they need to show exactly where they believe the Treasury and Department of Finance have got it wrong."

The Treasurer is preparing to release an economic statement, possibly next week.

"I've said the terms of trade, what the world is prepared to pay us for our goods, is falling, that has an impact," Mr Bowen told Channel Seven earlier today.

The update will include the cost of the Government's Papua New Guinea asylum seeker deal.

"We'll account for all our spending in the economic statement in an open and transparent way," Mr Bowen said.

The Treasurer reiterated the Government's commitment to return the budget to surplus in 2016-17.

"We've been very clear and consistent - balance then surplus - according to our strategy."