Two ACC clients have lodged formal complaints after confidential files went missing while their doctor was travelling on a bus.
The Accident Compensation Corporation says the doctor's bag containing the files was stolen or mistakenly taken by another passenger.
The corporation says police were immediately notified and the matter is being investigated.
It says the doctor has written to all the affected clients, two of whom have laid complaints with the Office of the Complaints Investigator.
The corporation says the matter is also being considered as part of a wider review of all ACC privacy issues, including the transportation of files.
The Office of the Auditor-General is also investigating.
The incident is the latest in a series of breaches. Last week, an ACC employee sent information about more than 100 business clients to the wrong people.