Loopholes that allow people on high incomes to claim Working for Families tax credits look set to be tackled in the 2010 Budget.
The Tax Working Group highlighted the issue in a report to the Government in January.
It estimates that income sheltered in trusts was worth about $300 million in tax revenue in 2007.
Revenue Minister Peter Dunne says the estimates are just the tip of the iceberg.
Finance Minister Bill English has already signalled a reduction of the top personal tax rate of 38% to the same rate as trusts, 33%, to deal with those who hide their income in trusts to avoid paying extra tax.
But trusts - and other measures - are also used to hide income so people are eligible for Working for Families tax credits.
Mr Dunne says the Government is considering the situation as part of changes to the tax system in the budget on 20 May.