Treasurer Josh Frydenberg has again dismissed modelling by left-leaning think tank, the Australia Institute, that showed the government's tax plan for high-income earners will cost the budget $77 billion.
But he and Prime Minister Scott Morrison have also repeatedly refused to say how much the tax cut in 2024 for those earning more than $180,000 will be.
Mr Frydenberg said the Australia Institute is aligned to the Labor Party, has union members on its board and is an organisation that has promoted death taxes.
"This is not an organisation that we take credibly," Mr Frydenberg told Sky News on Monday ahead of a National Press Club debate with his Labor counterpart Chris Bowen.
Mr Frydenberg said the cost to the budget to reduce the rate of tax from 32.5 to 30 cents in the dollar for those who are are earning between $45,000 and $200,000 is $95 billion, but decline to break that figure down any further.
Ahead of the debate, Mr Bowen has written a feisty letter to the treasurer, demanding a greater breakdown of the cost of the proposed tax cuts for people earning more than $180,000 each year.
Mr Morrison said on Friday said such a figure "hasn't been specifically set apart".
"If this is true, it indicates a grossly irresponsible approach to fiscal management from your government," Mr Bowen's letter said, requesting a more detailed breakdown from Treasury.
Australian Associated Press