The Parliamentary Budget Office (PBO) has estimated the Federal Government's corporate tax cut plan will cost $16.5 billion a year in a decade.

The budget proposal would see the company tax rate fall to 25 per cent for all firms in 2026-27.

The changes will be phased in over time, with small and medium-sized businesses to benefit first.

Government budget estimates show the changes would cost $2.65 billion over four years.

The Opposition used Question Time to repeatedly ask the Government about the absence of costings for the full 10-year period.

"Our plans are fully costed," Treasurer Scott Morrison said of the four-year estimate.

"Modelling details were released last night in accordance with the normal processes of the budget.

"A tax on businesses that employ millions of people in this country... [is] a tax on their enterprise, and a tax on that is a tax on growth and it's a tax on jobs."

The ABC has seen PBO office analysis tracking the full cost of the plan, showing it would cost $16.5 billion in year 26-27 or about 0.5 per cent of Gross Domestic Product (GDP).

Treasury estimated the tax cut plan would grow GDP by 1 per cent.