Repeatedly asked by Sky News' David Speers to name the number, the Mr Turnbull at one point said Treasury "has not identified the dollar cost of that particular item".

On Thursday, Prime Minister Malcolm Turnbull refused to provide the figure during an awkward interview with Sky News, or during sustained questioning from Labor in Parliament.

Mr Fraser said he had only been permitted to release the costing of two of the 11 prongs of the plan: reducing the company tax rate to 25 per cent, and increasing the threshold at which small businesses qualify for a lower tax rate.

One prong of the plan Mr Morrison did not permit Treasury to release was a move to increase the tax discount for unincorporated small businesses. The discount is currently 5 per cent and would increase to 16 per cent by 2026/27. Budget papers showed it would cost $450 million in the three years to 2019/20. Roger Brake, acting head of the Treasury's Revenue Group, confirmed Treasury had calculated that 10-year cost but had not been given the go-ahead to release it.

The $48.2 billion calculation does not include the 10-year cost of lifting the upper threshold of the 32.5 cent tax bracket to $87,000 from $80,000. That was estimated to cost $4 billion over the four-year forward estimates.

Senator Wong said it was clear the cost of the government's entire tax plan was "obviously going to be significantly more than the $48.2 billion".

The government argues the 10-year figure is immaterial because it is included in the Treasury's projection of the underlying cash balance, which shows the budget returning to surplus in 2020/21 and staying there.

Mr Morrison on Friday said it was not usual practice to put 10-year costings in the budget papers, but confirmed he had authorised Treasury to provide the information because it was "in the public interest".