The Federal Opposition has used the final Question Time before the election to continue its assault on the Government about the cost of its plan to cut company tax.

Prime Minister Malcolm Turnbull has refused to confirm the 10-year cost of the proposal to cut tax for all firms to 25 per cent over a decade.

The Parliamentary Budget Office (PBO) has estimated the proposal, which will be phased in over time and benefit small and medium-sized businesses first, will cost $16.5 billion a year in 2026-27.

However, during an interview with Sky News, Mr Turnbull would not confirm that figure, despite being asked more than a dozen times for an explanation.

"The Treasury has not identified the dollar cost of that particular item," Mr Turnbull said.

During a heated Question Time in the House of Representatives, Shadow Treasurer Chris Bowen accused the Government of trying to "cover up" the cost.

"What a budget and what a launch of an election campaign," Mr Bowen said.

"Its centrepiece measure is a fraud on the Australian people.

"The Prime Minister's off to see the Governor-General in the next couple of days and he launches an election campaign by not being honest with the Australian people."

But Mr Turnbull said it was not appropriate to release the costs for the next 10 years.

"It is not the practice of the Government or indeed governments to release itemised elements of the medium-term projections beyond the forward estimates," Mr Turnbull said.

"The Charter of Budget Honesty Act stipulates the budget papers must contain details of policy and estimates for the budget year and the following three years."

Opposition asks PM to confirm economist's $55b estimate

Budget economist Chris Richardson has estimated the proposal would cost $55 billion over the decade.

Opposition Leader Bill Shorten asked several questions of the Prime Minister on the issue and asked him to confirm that forecast.

"Prime Minister, Australians want to know," Mr Shorten said.

"Is he right, is it $55 billion? If not, what is the 10-year cost?"

Mr Turnbull refused to give specific details, saying Mr Richardson "may or may not be correct".

"The estimations and forecasts and calculations of experienced economists like Dr Richardson are always taken into account by the public and indeed by members of this house," he said.

"I took care to say that I am not confirming or commenting on his estimate, other than to say he is a former Treasury economist and we should heed his advice. It may or may not be correct."

The ABC has seen PBO office analysis tracking the full cost of the plan, showing it would cost $16.5 billion in a decade or about 0.5 per cent of GDP.

Treasury estimated the tax cut plan would grow GDP by 1 per cent.

Labor would raise taxes, Morrison says

Treasurer Scott Morrison also refused to say how much the corporate tax plan would cost over 10 years, but denied estimates it would cost $55 billion.

He was adamant the proposal was "affordable".

"Every measure across the medium term, across the short term in the budget and forward estimates is affordable," Mr Morrison said.

He said the Opposition would raise taxes if elected to Government.

Mr Bowen pounced on the remarks in Parliament.

"It takes a particular level of incompetence to bring down a budget with a centrepiece uncosted," he said.

He described the budget as a "fraud on the Australian people by having a centrepiece without a cost attached".