Treasury officials have revealed the Government's plan to cut the company tax rate to 25 per cent will cost $48.2 billion over 10 years.

The estimate from the Treasury Secretary comes after the Opposition pounced on the Coalition's refusal to outline the full cost of the proposal.

Treasury Secretary John Fraser told Senate Estimates it is not "standard practice" to release costings beyond four years, but said the 10-year company tax plan would cost tens of billions of dollars.

"The cost of these measures to 2026-27 is $48.2 billion in cash terms," Mr Fraser said.

"I note that as with all tax projections over ten years these costings have considerable uncertainty attached to them.

"I also note that the medium-term economic projections in the budget assume significant ongoing economic reforms."

Mr Fraser said the Treasurer Scott Morrison had authorised the release of the projection, but only after continued pressure from the Opposition.

He made the comments during questioning from Labor Senator Penny Wong, detailed in the following exchange:

Wong: Can you tell me when the decision was made to put the ten-year costs of the company tax cuts into your opening statement? Fraser: Late yesterday afternoon. Wong: Who made the decision? Fraser: The Treasurer. Wong: Was it at your suggestion that the ten-year cost be included? Fraser: No. Wong: So the Treasurer determined to put it in? Fraser: Correct. Wong: Was this discussion before or after question time? Fraser: It was about six o'clock ... roughly.

Treasury officials have also confirmed only two of the 11 measures outlined in the budget's ten year enterprise tax plan had been costed.

Turnbull lied about tax cut proposal: Opposition

The Federal Opposition is accusing the Prime Minister Malcolm Turnbull of lying about the corporate tax cut proposal.

"Mr Turnbull told a blatant political lie yesterday when he said Treasury hadn't worked out the cost of that item," Opposition Leader Bill Shorten said, referring to an interview the PM gave on Thursday.

"The Prime Minister of Australia looked down the barrel of the camera and he lied to Australians about whether Treasury had costed his expensive plan to give tax cuts to multinationals.

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

Later in the same interview on Thursday Mr Turnbull said Treasury had modelled the figure.

Education Minister Simon Birmingham told Sky News Labor had resorted to "personal smear and attack" and was taking the Prime Minister out of context.

"Of course the Prime Minister is quite right that in the budget papers Treasury has not identified that cost, so it's a perfectly accurate statement," he said.

"It was a completely accurate reflection of what's in the budget papers."