Australia's capital gains tax discount is set to cost the budget nearly $250 billion over the next ten years, according to new data from the Parliamentary Budget Office.

The figures show that retirees and high-income earners are the main winners under the current system. This financial year alone, the wealthiest 1% of taxpayers are expected to take home nearly 60% of the total benefit.

The Greens commissioned the report to put pressure on the federal government to overhaul the policy. While no official plans have been announced, senior Labor officials have suggested that changes could be on the table in the upcoming budget.