The Inland Revenue Department sent the personal information of 6300 people to the wrong person in the past year.
Revenue Minister Peter Dunne said most of those affected will never know because the breach was not considered serious enough to contact them
But in the most serious cases, 638 people had details like their addresses and tax numbers released, making them vulnerable to identity fraud.
Mr Dunne said the affected people have been contacted and the problems were due to human error.
Better checks are in place now, including a rule that no email is sent with multiple identifying details, he said.