The Bank of New Zealand has given $4 million back to 90,000 customers who were charged foreign currency fees by mistake.
The error was uncovered during an internal review by the bank of its documents which found some customers had been told they would not have to pay foreign currency service fees.
The charges affected VISA and personal loan customers whose terms and conditions have since been amended to include the charges.
BNZ corporate affairs head Mark Watts said the banks' processes have worked well.
The Banking Ombudsman said she is happy with how BNZ is dealing with the mistake and has not heard of any other banks having similar issues.