Prime Minister Malcolm Turner in a TV interview refused to confirm the cost to taxpayers of his Government's plan to cut company tax to 25 percent over the next decade.

The PM sat down with Sky News host David Speers, who repeatedly pressed Mr. Turnbull to come up with a number - but Malcolm would only say that the budget is expected to be back in the black by 2020 - 2021. 

"The Treasury has not identified the dollar cost of that particular item," said the Prime Minister. "The cost of the plan is set out in the medium-term outlook and shows the budget returning to balance."

"I don't understand what the cost is," Mr. Speers insisted, "What's it going to cost taxpayers to cut the company tax rate to 25 per cent?" he asked.

"What it ensures is that we'll have stronger jobs and growth," Mr Turnbull replied.

The government's proposal would decrease the tax rate on small businesses to 27.5 percent.  The threshold to qualify for that lower rate would gradually increase to turnover of $1 billion.  After that, the tax rate for all companies will be stepped down to 25 percent.  Phasing it in over time, the Parliamentary Budget Office (PBO) believes it would cost $16.5 billion a year in 2026 - 2027.

But the PM's failure to own that number provided fodder for his opponents in Parliament.

"It takes a particular level of incompetence to bring down a budget with a centerpiece uncosted," said Shadow Treasurer Chris Bowen, who lambasted the budget as a "fraud on the Australian people by having a centerpiece without a cost attached".