The Accident Compensation Corporation is apologising unreservedly for a botched mailout in which thousands of confidential reports were sent to wrong addresses.
ACC says 2000 of the 15,000 claim reports it sends to businesses each month were posted with the wrong covering letters attached.
The letters were wrongly collated by a mailing company, which the corporation has declined to name.
Another 7000 incorrect forms were intercepted before they were sent out.
General manager Keith McLea says ACC realised there was a problem on Tuesday morning when a business called to say the names on the form did not match its employees.
Dr McLea says the letters do not contain age, address or date of birth information, but do detail the employee's name and injury.
Assistant Privacy Commissioner Katrine Evans says ACC has acted quickly and is trying to get all of the letters returned.
However, she says it is a serious privacy breach because the letters contain potentially embarrassing medical information.
ACC says the mailing company will cover the cost of retrieving the letters.