Five South Auckland Work and Income staff have been fired for inappropriately accessing clients' files.
The employees, all from the Manukau office, were fired for accessing information on family and friends.
WINZ said there were "many breaches" by the staff and a national investigation has now been launched.
Work and Income head Janet Grossman said it was "vital New Zealanders have confidence in the integrity of our staff and the welfare system".
"I'm conducting this review of the way staff handle client records because I want to confirm that these breaches are confined to this office.
"The staff members were fired for accessing the information of their family and friends. They were also guilty of breaching our email policy."
She said she was "appalled and utterly gutted" by their actions.
"These people have let us down badly. Their actions cast a shadow over our honest and hard working staff who understand that client privacy is sacrosanct."
Grossman said the investigation at the Manukau office was ongoing and the national review was expected to be concluded in February.
"Work and Income has zero tolerance for staff who breach the privacy of clients. Our Integrity Unit regularly conducts random checks of our systems, to detect such breaches.
The Privacy Commissioner has been informed and each client whose privacy has been breached is being contacted individually.
"Each and every day Work and Income staff are entrusted with the private and personal information of more than half a million New Zealanders. There is never any excuse for accessing a client's file without a legitimate work-related reason."
Work and Income staff sign a copy of a Code of Conduct and a declaration acknowledging their responsibility to protect privacy.
The agency has asked that anyone with privacy concerns contact them.
"I am committed to ensuring that we maintain the trust of the New Zealand public" Grossman said.
- Auckland Now
Is there enough support for new migrants once they arrive in New Zealand?