The Abbott government's long-term forecasts for surpluses have been bolstered by an unheralded accounting change that includes for the first time earnings from the Future Fund in the budget bottom line.

In one of the most important projections in the budget, Treasury said this year's $41 billion deficit will narrow close to zero by 2020 before turning into six consecutive surpluses averaging 0.5 per cent of gross domestic product through to 2026.