The ANZ bank double charged 200,000 non-ANZ customers to the tune of $45 million, after a computer glitch duplicated 400,000 MasterCard transactions.
The glitch affected non-ANZ customers who had used MasterCard credit cards on ANZ Eftpos machines between Friday May 12 and Monday May 15.
ANZ spokesperson Paul Edwards said the glitch was caused by human error during a routine maintenance check.
Edwards said the mistake had been corrected and refunds issued with appropriate interest.
"We are in the process of putting in additional checks and balances to make sure this doesn't happen again," he said.
Edwards said all affected customers should have had their money returned by last Friday morning.
"We're not happy the event occurred but we're happy with how the issue has been cleaned up quickly," he said.