Medical notes of 118 people who received physiotherapy at Gisborne Hospital between 2012 and 2015 have gone missing.
In a statement released by the Hauora Tairawhiti District Health Board, it was said the missing notes may include the details of hand treatments and referrals from a doctor, but do not include the patients' full medical records.
Communications manager Toni Lexmond says the Board cannot rule out that they could be viewed externally.
The incident was discovered after an audit, and a full investigation followed.
The statement said the missing notes "can all be attributed to the treating therapist, a previous employee of Hauora Tairawhiti".
"This person has been assisting with the investigation. Most of the missing notes were located, but 118 have not been found."
A family member of one of the affected patients, who wishes to remain anonymous, says he was concerned this incident had occurred and by the potential for others to view the missing information.
Shared Services Clinical Care Manager Kate Mather said they "have done all we can to recover the missing notes and to make sure this never happens again".
The physiotherapist has been referred to their professional body, who are also conducting an investigation.
The Privacy Commission and the Ministry of Health have been notified.