Labor seeks answers on Turnbull tax plan

Malcolm Turnbull faces an election-eve credibility problem after admitting his 10-year company tax cut plan had not been costed by Treasury.

However the prime minister said it was not unusual, as Treasury provided costings for the next four years and an "outlook" for years beyond this.

The federal opposition seized on the admission, saying taxpayers were entitled to know how much it would cost and pointed to previous Labor budgets in which 10-year costings were provided for the Gonski schools funding and the national disability insurance scheme.

The coalition wants to make economic management a key issue for the July 2 election, which Mr Turnbull will call this weekend.

Mr Turnbull has routinely criticised Opposition Leader Bill Shorten over big spending promises, which the government says amount to a $62 billion budget blowout under Labor over the next four years.

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."

The outlook showed a return to surplus in 2020/21 and surpluses beyond that period, he said.

Shadow treasurer Chris Bowen twice unsuccessfully attempted to have parliament pass 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".

Asked whether a $55 billion estimate by economist Chris Richardson was accurate, Mr Turnbull said: "He may well be right."

However, he told parliament later the estimate would depend on Mr Richardson's assumptions.

Treasurer Scott Morrison said the $55 billion figure was wrong.

"This is completely within the affordability markers we have set," he said.

Finance Minister Mathias Cormann told reporters in Canberra the tax plan was "fully costed and fully funded".

Quizzed by Labor in parliament over the figures, Mr Turnbull said Mr Shorten was being "childish", as four-year estimates had been the standard of every budget since 1998.

Mr Shorten said the coalition had tripled the deficit since 2013 and cut funding for schools, hospitals and pensioners.

"The budget has the wrong priorities and it also has all the old unfairnesses," he said.

The opposition rejected the $62 billion blowout figure, as it included a number of errors including $20 billion in foreign aid spending which the opposition has only pledged to independently review in government.