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.