Thousands of email addresses of nurses' union members have been leaked after the organisation was targeted by a scam.
The New Zealand Nurses Organisation said most of its nearly 47,000 members were affected.
The data breach happened on Tuesday as a result of a phishing scam, used to obtain information by making recipients believe they are responding to a legitimate request.
Acting chief executive Jane MacGeorge said a staff member opened an email that appeared to be from the chief executive.
"We've immediately apologised to our members and alerted them."
The union said the Department of Internal Affairs had requested Yahoo close the fake email address.
It has met with police and the Ministry of Health over the breach and was was working with the Privacy Commissioner to support its members.
A call centre was available to provide advice.