According to a document on the integrity of GST systems in small to medium enterprises issued by the Australian Tax Office recently:

– In 2009-10 the ATO identified on average $AUD42,000 of under reported GST for every small to medium enterprise they investigated.
– A quarter of all discrepancies arose in the retail sector and 19% in wholesale trades.
– 32% of discrepancies arose from mistakes in GST returns.
– 27% arose from technical misunderstandings.
– 24% were system related issues.

Not surprisingly the Australian Tax Office recommend regular GST systems checks.

It wouldn’t be surprising if similar results are being achieved by the New Zealand IRD.