The Coalition government's personal tax cuts for middle and high income earners will cost $30 billion a year, new independent costing of the plan shows amid warnings they may drive the budget back into deficit.

As research by the Grattan Institute warns the tax cuts may not be affordable, figures compiled by the Parliamentary Budget Office show the third stage of the government's budget tax measures will come at a substantial cost to the budget bottom line.

The stage three tax cuts promised by Prime Minister Scott Morrison will cost the budget $30 billion a year according to costings by the Parliamentary Budget Office. Credit:Alex Ellinghausen

Labor is opposed to stages two and three of the government's tax plan. The costing by the PBO, however, only looks at the budget impact of stage three under which a 30 per cent tax rate will cover all workers earning between $45,000 and $200,000 a year.

The plan builds upon last year's move to axe the 37 per cent tax rate, with the government estimating 70 per cent of workers will find themselves on the new tax rate that is slated to start from July 1, 2024.