Well-off Australians are slashing their personal tax bills at a cost of billions of dollars a year to other taxpayers through the widening use of secretive family trusts.

But while trusts are flourishing, and the government is battling a big budget deficit, neither the Coalition nor Labor has tackled what critics describe as the "sacred ground" of tax minimisation.

Unpublished figures provided exclusively to Fairfax Media by the Australian Taxation Office reveal almost 643,000 discretionary trusts (most family trusts are this kind) in Australia in the 2014-15 financial year, the most recent year for which figures are available. That is almost twice the number of 20 years earlier, an increase that far outstrips population growth.

The increase is likely to have been sharper in the two years since 2015 as other popular tax minimisation options – superannuation and negative gearing in particular – come under heavy political pressure, and financial planners and accountants market trusts as an alternative.