The Office of the Privacy Commissioner has opened an investigation into revelations that an Accident Compensation Corporation staff member emailed a client details of more than 9000 claims.
In a statement on Wednesday afternoon, the office said it was looking into the breach and ACC's standards for securing personal information.
It says the client who received the information does not appear to have passed it on to anyone else.
Earlier, the Green Party said it had asked the Privacy Commissioner to investigate what it says are systemic privacy breaches at ACC.
Greens ACC spokesperson Kevin Hague says he lodged a formal complaint with the commissioner because it is not the first significant breach of privacy at ACC.
Mr Hague says ACC has a track record of privacy breaches and it should have had good systems in place by now.
He says he has received more than 50 emails and phone calls from concerned ACC claimants seeking an investigation into the latest breach.
Late on Wednesday afternoon, the Office of the Privacy Commissioner issued a statement saying it had opened an investigation into the incident, and into ACC's standards for securing personal information.
Chief executive Ralph Stewart has publicly apologised for the breach and says the corporation will phone all 6748 clients affected to apologise.
Mr Stewart says a review is now underway and will be completed in two stages.
He says a privacy breach report to be provided to the Privacy Commissioner will be completed by the end of April.
Mr Stewart says ACC, with independent support, will review its business process in relation to privacy and release a report before the end of September.