Inland Revenue has apologised for mistakenly releasing the personal details of nearly 30 people.
The incident happened last week and an internal investigation has indicated that it may have been caused by human error.
An IRD deputy commissioner, Arlene White, says the recipient of the information has been contacted and they are working to get the information returned.
She says the processes that led to the mistake are now being reviewed as a matter of urgency.
The people whose information was sent out in error have received apologies and the department intends to carry out an investigation.
The incident has also been referred to the Privacy Commissioner.
