Inland Revenue has apologised after personal details for just under 30 customers were incorrectly released.
Deputy Commissioner Service Delivery Arlene White said a preliminary internal investigation indicated last week's incident may have been caused by a manual handling error.
"We have contacted the recipient of this information and our highest priority is the return of the information,'' Ms White said.
"We are also contacting the customers whose information has been released to apologise and letting them know the steps we are taking to remedy the situation.''
Ms White said Inland Revenue was undertaking a full investigation and the incident has been referred to the Privacy Commissioner.
"Inland Revenue is committed to ensuring that all personal information is secure and customer privacy is maintained at all times.
"The mail handling processes that may have led to the error are being reviewed as a matter of urgency.
We will also be talking with our staff about our processes for handling correspondence and the need for extreme care to be taken at all times.
"We process in excess of 25 million transactions and correspondence every year and regrettably errors can occasionally occur.''