The refunds work out at about $375 each, and come after the bank failed to update customers about changes to its mortgages quickly enough, and in some cases, used the wrong information.
BNZ said no customers were overcharged during the incident, simply that it failed to notify some customers on time. It affected less than 1% of customers.
The error dates back to 2015 and relates to the Credit Contracts and Consumer Finance Act. CCCFA was introduced to simplify the relationship between lenders and borrowers.
Letters will be sent to BNZ customers this week, with money expected to land in customer accounts today.
BNZ CEO Angie Mentis said: “No customers were overcharged, but we didn’t get it 100 per cent right. While mistakes do happen from time-to-time, finding those mistakes and fixing them is simply the right thing to do for our customers."