PM defends tax plan cost dodge

Prime Minister Malcolm Turnbull has defended the time it has taken to reveal the $48.2 billion price tag attached to the centrepiece of his first budget.

Treasury secretary John Fraser finally revealed the cost of the government's 10-year tax plan in a Senate Estimates hearing on Friday morning, after Mr Turnbull and Treasurer Scott Morrison had refused to detail the figure since handing down the budget on Tuesday night.

"The projections for the enterprise tax cuts - the company tax cuts - were of course calculated by Treasury, they made those assumptions," Mr Turnbull told reporters in Sydney on Friday.

"And it's appropriate that the authors of those assumptions were the people discussing them and detailing them, and of course being questioned on them, by the senators."