The coalition's campaign to cut five cents from the fuel excise would blow a big hole in the budget and might not cut petrol prices, Treasurer Wayne Swan says.
"It's by no means certain it would ultimately lower petrol prices at all," Mr Swan told Network Ten.
He accused Opposition Leader Brendan Nelson of not properly costing the fuel excise proposal.
"He and Mr Turnbull are prepared to blow a $22 billion hole in the surplus," Mr Swan said.
"They want to spend $22 billion more? I mean, they have no economic credibility any more."
Mr Swan acknowledged climate change - through the introduction of an emissions trading scheme (ETS) in 2010 - would cause petrol prices to rise.
"It is true there will be some impact as we move through with an emissions trading system on the price level of a number of products, but the cost of not acting is far greater than the cost of acting," he said.
Mr Swan said the government would discuss the ETS, which is still being designed, with the Australian community.
Dr Nelson last week proposed in the budget reply speech that a coalition government would spend $1.8 billion to cut the fuel excise.
But on Friday he admitted this could blow out to $2.5 billion if the government was forced to refund $750 million in GST to the states, although he refused to say whether this would occur.
Dr Nelson said the proposal to cut fuel excise would be costed well before the next federal election in 2010.
"As is always the case with the Liberals and Nationals, all of our policies are and will be fully costed and budgeted," Dr Nelson told ABC Television on Sunday morning.
"We will make it absolutely clear, what our budgetary forecast is, we'll make it absolutely clear where this $1.8 billion will be coming from.
"All of our policies, all of our costing will be put up for analysis well before the next federal election."
Opposition treasury spokesman Malcolm Turnbull said the proposal had not yet been costed because the coalition were in opposition.
"There is no suggestion of a saving there... and it is simply because ... we are in opposition, if we were in government we would find savings," Mr Turnbull told the Nine Network.
Mr Turnbull said the coalition would cost the proposal before the 2010 federal election.
"When we go to the next election we will have a proposal to cut the fuel excise."
He said the cost would be taken out of the budget surplus if the coalition was unable to find savings elsewhere.
"What we are saying is that as a mater of principle we will fund that out of savings elsewhere."
"(But) if nothing else is done it will obviously come out of the surplus."
