In another embarrassing privacy blunder by a government agency, Inland Revenue has admitted it accidentally released nearly 30 customers' information to a single client last week.

Deputy commissioner of service delivery Arlene White said a preliminary internal investigation found a manual handling error may be to blame for correspondence being sent to the wrong person.

Inland Revenue has contacted the sole recipient and is seeking the return of the information.

It was also contacting customers whose information was released to apologise "and letting them know the steps we are taking to remedy the situation", Ms White said.

"We process in excess of 25 million transactions and correspondence every year and regrettably errors can occasionally occur," she said.

"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."

Ms White said Inland Revenue was undertaking a full investigation and has referred the incident to the Privacy Commissioner.

The bungle follows a major privacy breach by the Accident Compensation Corporation, revealed earlier this year, where the corporation accidentally emailed a spreadsheet with details of 6752 ACC clients to a single client last August.