Westpac New Zealand is forking out millions of dollars after it overcharged more than 30,000 customers.
The bank found it had overcharged customers on fees for some of their former accounts, RNZ reports.
It said the mistake could cost almost $6 million.
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The overcharging related to customers with Access transaction accounts which the bank phased out last year.
Westpac has apologised to customers for its mistake, in which customers with those accounts didn't get the correct discount on fees they paid.
The fee waiver for the accounts had been miscalculated and applied incorrectly, the bank admitted.
Customers will be getting a refund, plus interest, over the next week.