Figures released today show the federal budget surplus is almost $4 billion greater than estimates published in the budget papers, a leading think tank says.

Labor unveiled a surplus of $21.7 billion when it handed down its first budget in 13 years in May.

But the Australia Institute says figures published today by the Australian Bureau of Statistics (ABS) show the surplus was actually $25.1 billion.

David Richardson, a research fellow at the Australia Institute, says the 2008-09 budget papers failed to take into account earnings from the Future Fund.

"The 2008 budget corrected most of the accounting distortions introduced by the Howard government, but it still failed to bring the earnings of the Future Fund to account,'' he said.

"The ABS has now corrected for that,'' Mr Richardson said.

"It seems curious that the Labor government has corrected the budget figures by including the $47 billion GST but ignored the smaller $3.5 billion earnings of the Future Fund,'' he said.

"It's about time governments used the true figure rather than making adjustments for political purposes.''

AAP