Prime Minister Kevin Rudd has accused the opposition of economic irresponsibility in its response to the federal budget.

Opposition Leader Brendan Nelson last night delivered his budget reply speech, proposing that a coalition government would spend $1.8 billion to cut the fuel excise.

• Cut petrol excise by 5 cents a litre
• Block Labor's proposed alcopop tax increase
• Block increase to Medicare surcharge income threshold
• Lower capital gains exemption to 5 years for small businesses
• Lift entry requirements for teacher training
• Increase carers benefits

But Dr Nelson today 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.

Visiting Brisbane today to announce $10 million for the Gateway Upgrade Project over the Brisbane River, Mr Rudd said the opposition was being irresponsible with the $22 billion budget surplus.

"This is economy irresponsibility 101," Mr Rudd told reporters.

"What Dr Nelson and Mr Turnbull have done is conduct a $22 billion raid on the surplus which means upward pressure on inflation and upward pressure on interest rates."

Mr Rudd rejected opposition claims the budget was a raw deal for pensioners.

"I think you'll find that what we're delivering in the budget is at least equal to what those opposite are offering," he said.

Mr Rudd said Dr Nelson had admitted to putting together his budget reply without costing the coalition's proposals.

"This is an essential requirement for national political leadership," Mr Rudd said.

"If you are putting together a budget you've got to cost it - you can't just hope one day that it might all add up."

Mr Rudd said the overall impact of the government's tax changes, education refund, childcare rebate and benefits for pensioners and carers totalled around $46 billion.

"These are all significant additions to the family budget, therefore we believe that's the responsible way to go," he said.

"What Dr Nelson has done instead is say: `I'm about to, on behalf of the Liberal Party, trash the Liberal Party's economic credibility by raiding the surplus to the tune of $22 billion, all
in the cause of economic populism'."

NRMA calls on opposition to develop wider petrol policy

The federal opposition should support the government's FuelWatch price monitoring scheme before considering a cut in the fuel excise, a leading motoring group says.

"Dr Nelson hasn't supported FuelWatch which we think is critical to make sure motorists get a fair deal," NRMA motoring president Alan Evans said today.

FuelWatch, a federally-backed scheme due for introduction in December, will let motorists know where the cheapest petrol is being sold.

Mr Evans also queried what would happen to road funding if less revenue was coming in from the fuel excise.

"Our view is that we want to be assured that any reduction in excise would not see a reduction in funding on roads."

Mr Evans called on the opposition to develop a wider policy on petrol.

"It's not a comprehensive policy, it might seem a bit opportunistic."

Cutting petrol price not the answer says analyst

Analysts are divided about the worth of Opposition Leader Brendan Nelson's pledge to cut the fuel excise by five cents a litre.

CommSec equities analyst Savanth Sebastian said there was not much thought behind the proposal.

"The reason being is that we continue to see that the rally in fuel prices is not just constrained to Australia," he said.

"It's on the global scale and the fact remains that if you start cutting these sorts of taxes all you're doing is ... just increasing investor demand for a particular commodity.

"It's likely those gains will be erased."