The final stages of tax cuts promised by Scott Morrison's government are expected to cost the Australian purse almost $30 billion each year by 2030.
The Parliamentary Budget Office - the nation's independent policy costing agency - had laid out the figure in a preliminary assessment of the changes.
The federal Liberal-National coalition has already legislated staged tax cuts, the first of which will kick off from the middle of this year, with further reductions in 2022 and 2024.
But in its latest budget last week it built upon the plans, pledging to double a tax offset for Australians on low and middle incomes from this year and drop the 32.4 per cent tax rate to 30 per cent from mid-2024.
The latter stages of the plan that come into effect by mid-2024 are a point of contention between the government and Labor.
That's because Labor says it would keep the legislated tax cuts planned to come into effect this year - with some extra elements - but is not supportive of the second stage and has vowed to abolish the third.
The PBO says not going ahead with the parts of the coalition's plan that would come into effect by July 2024 would save the budget $18.7 billion in 2024/25.
The saving would gradually increase each year, until it hits $29.8 billion in 2029/30.
Shadow treasurer Chris Bowen says that is a "major structural dig" into the budget over the long-term for changes that will largely flow to Australians on high incomes.
Once the coalition's changes are fully rolled out by 2024/25, the tax cuts will mean someone earning $200,000 will be able to keep an extra $11,640, while someone earning $30,000 will get an extra $255.
"It is nor fair or responsible to lock-in billions of dollars of tax giveaways that disproportionately benefit a relative few - and so far into the future," he said in a statement.
Treasurer Josh Frydenberg has said the high-end income tax cuts are about boosting the economy.
"It's not the government's money, it's the people's money," he told reporters last week.
