Treasurer Wayne Swan will dip into the pockets of the rich to deliver Labor's first budget surplus in 13 years, arguing it is the right economic decision for the times and will ease the interest-rate squeeze on working families.

Mr Swan's fifth budget, to be handed down on May 8, comes as Labor seeks a boost over the coalition in the polls and Prime Minister Julia Gillard faces renewed leadership speculation over her handling of the Peter Slipper and Craig Thomson affairs.

The treasurer has the daunting task of turning an estimated $40 billion deficit in 2011/12 to a surplus of $1.5 billion in 2012/13, amid falling company tax revenues and rising demand for government services.

Mr Swan argues a surplus is appropriate for an economy returning to trend growth of 3.25 per cent and will be the nation's best defence against further global uncertainty.

He also hopes it will give the Reserve Bank of Australia, which cut official interest rates by a hefty 50 basis points a week ahead of the May 8 budget, the ability to keep rates low.