Work and Income will reimburse tens of thousands of people after it underpaid them by nearly $30 million.
A coding error meant about 49,000 former and current clients, dating back to 1993, were underpaid their accommodation supplement.
Minister of Social Development Anne Tolley said clients would start to receive reimbursements this week.
Ms Tolley said the average reimbursement was about $590, with 48 people to get $10,000 or more.
The highest amount was $28,500, which would be paid to a person on NZ Super.
Former clients can check their eligibility on the ministry's website.