Labor has accused Malcolm Turnbull of incompetence after the prime minister admitted his 10-year tax plan had not been costed by Treasury.
Mr Turnbull has hammered Opposition Leader Bill Shorten over big spending promises made by previous Labor governments that were not properly funded, making balancing the budget harder for the coalition.
The prime minister was asked on Sky News on Thursday how much it would cost to lower the company tax rate to 25 per cent over 10 years - a plan set out in Tuesday's budget.
"The Treasury has not identified the dollar cost of that particular item," Mr Turnbull said.
"What it has done is set out a medium-term outlook which takes account of the company tax cut and all the other tax arrangements."
Shadow treasurer Chris Bowen unsuccessfully attempted to have parliament debate a motion condemning the prime minister for what amounted to a "fraud".
"It takes a particular level of incompetence to bring down a budget with a centrepiece that is uncosted," Mr Bowen said.
Mr Turnbull said the corporate tax cut - which was estimated to permanently boost economic growth by more than $16 billion a year - was "clearly affordable".
However, he declined to "unpick every single line item" of Treasury's economic assumptions.
Asked whether a $55 billion estimate by one economist was accurate, Mr Turnbull said: "He may well be right."
Asked the same question on 3AW Radio, Treasurer Scott Morrison also declined to put a cost on the tax plan saying it was included in the projected underlying cash balance.
"This is completely within the affordability markers we have set," he said.
Ahead of a July 2 election to be called this weekend, the Liberals are making economic leadership the focus of their campaign.
"We need wise and strong economic leadership," Mr Turnbull said.
Mr Shorten, who will deliver his budget reply speech to parliament on Thursday night, told caucus members the government's priorities were wrong, while the budget deficit had tripled.
"Budgets are meant to help communities build and grow," he said.
