The Inland Revenue Department is not doing enough to prevent child support debt from building up, says Auditor-General Lyn Provost.
More than $1.5 billion of child support payments went unpaid last year.
Ms Provost examined how well the department was managing the collection of child support debt.
She says Inland Revenue should do more to prevent any build-up of debt in the first place.
Ms Provost says it should create a scheme that more parents understand and support, and look at new international agreements that would make it easier to collect child support from overseas.
Ms Provost says outstanding payments are expected to reach about $7 billion by 2018 if the current system continues unchanged.
Revenue Minister Peter Dunne says he will put proposals to Cabinet on child support, which he says will give the 20-year-old system a shake-up.