AUSTRALIA'S top earners nearly doubled their income during the Howard government years, new figures have revealed, providing ammunition to Treasurer Wayne Swan as he prepares to slug the rich in the federal budget.

An analysis prepared by Treasury shows that families with an earner in the top 3% of taxpayers have enjoyed an 85% increase in their disposable incomes since 1996 — about 1.7 times the rise for families with an average income earner.

The top 3% have also seen their average rate of tax fall by 6.2 percentage points in that period, almost twice the 3.3 percentage point reduction for someone on average earnings.

The analysis, prepared at the Government's request and obtained by The Age, could be used by Mr Swan next week to defend a squeeze on the rich in a budget designed to tackle inflation with big spending cuts.