The Turnbull government's signature company tax cut will cost an eye-watering $65.4 billion over 10 years from July 1, 2017, the Prime Minister has confirmed, a more than $15 billion rise from the initial near $50 billion cost.

The company tax cut package, designed to slash tax for all companies to 25 per cent by 2026-27, was initially costed at $48.2 billion over 10 years from July 1, 2016.

About $24 billion worth of that $50 billion tax cut has been passed so far by Parliament with the rest of it blocked by the Senate.

The rise in the cost of the tax cut to $65 billion is because of the addition of a new "10th" year, 2027-28, to the forecast period in which the full company tax cut – from 30 to 25 per cent – would be in place for all companies, if the government can pass the required laws.