Treasury counters Turnbull tax comment

Labor has accused Malcolm Turnbull of a "blatant political lie" over the government's 10-year company tax plan, days before the election starting gun is fired.

The prime minister told Sky News on Thursday Treasury had "not identified the dollar cost" of the cut in company tax to 25 per cent over a decade.

But Treasury secretary John Fraser told a Senate estimates hearing on Friday the figure had been known weeks ago and the hit to the budget came to $48.2 billion over 10 years.

Mr Fraser revealed Treasurer Scott Morrison had asked him to unveil the figure at the hearing, following a final day of parliament on which the government took flak over the hidden number.

Mr Turnbull told reporters on Friday it was appropriate that Treasury detailed the costings.

"As I said yesterday, the budget papers do not identify the dollar cost, the items in the 10-year projections," he said in Sydney.

Mr Turnbull said the budget papers released on Tuesday laid out details over four years but Treasury also provided a "medium term outlook" for a longer period.

Opposition Leader Bill Shorten told reporters in Canberra the prime minister had misled voters.

"Mr Turnbull told a blatant political lie yesterday when he said that Treasury hadn't worked out the cost of that item," Mr Shorten said.

"Less than 24 hours later, Treasury is now producing costings which Mr Turnbull said did not exist."

Mr Fraser noted it was not standard practice to release 10-year figures, even though Treasury had costed a number of tax and superannuation policies over 10 years.

Mr Morrison defended not producing the numbers earlier.

"I wasn't going to do Bill Shorten's homework for him," he said.

The Labor leader in his budget reply speech on Thursday estimated the benefit of axing the plan would be $49 billion.

Labor supports a tax cut for small businesses with a turnover of less than $2 million a year.

Meanwhile, Treasury cast doubt on a $6 billion budget saving proposed by Labor.

Mr Shorten outlined in his budget reply speech a move to save $6 billion by introducing a loan cap of $8000 a year in the VET FEE-HELP program - which provides loans for vocational education students.

Under Liberal questioning at a Senate estimates hearing on Friday, senior Treasury official Michael Brennan said he could not comment on the details of the policy, but any limit placed on the size of this type of loan "would have a positive impact on the fiscal balance".

"But over time, probably a slightly negative impact on the underlying cash balance," Mr Brennan said.

The Parliamentary Budget Office, which costed Labor's policy, said it would provide savings over the next four years as well as the decade.

Parliamentary Budget Officer Phil Bowen said the saving "reflects lower loan balances and therefore reduced public debt interest payments required to fund the concessional interest subsidy and bad debts associated with these loans".