Queues formed at cash machines in the Australian city of Sydney after they malfunctioned and began giving out large sums of money.
More than 40 Commonwealth Bank cash machines across the city were affected by the problem.
Officials said they were operating in stand-by mode, so could not identify the account balances of customers.
That meant that in some cases customers who had little or no money were able to withdraw large sums of cash.
The technical glitch reportedly lasted for about five and a half hours.
Witnesses spoke of a queue of as many as 50 people at one particular cash machine in a Sydney suburb as word got out, Australian media said.
Problems were also reported in Melbourne.
"As a result of our problems today, some of our customers have gone to our ATMs and they've been able to withdraw more funds than they had in their account and therefore they've overdrawn their account," spokesman Bryan Fitzgerald told the Australian Broadcasting Corporation.
"We have a record of every transaction that's been made today and we'll be following up with our customers to make sure that we recover the funds they've overdrawn."
Police warned that it was an offence to keep money dispensed accidentally from a cash machine and urged people to return the cash.
"They should also realise that ATM locations are covered extensively by surveillance," Det Supt Col Dyson of New South Wales fraud squad said in a statement.