Queues of people have formed at ATMs in Sydney that are reported to have been spewing out large sums of cash.
Customers say some of the Commonwealth Bank machines have been paying out thousands of dollars.
Police are warning, however, that it is a criminal offence to keep any excess money dispensed and ATM locations are covered by surveillance cameras.
Some of the machines are said to have produced 10 times the amount of money sought by customers.
New South Wales police say they have received "numerous reports" of extra cash being dispensed from more than 40 ATMs.
Callers told the ABC of some cardholders withdrawing several thousand dollars, with a queue of up to 50 people seen at the Redfern branch.
But a bank spokesperson says the affected machines are "not accidentally or randomly dispensing cash; our ATMs are currently operating and have been operating in standby mode.
"That means the ATM... can't identify the customer's account balance. Some have deliberately withdrawn more money than is in their account ... and we will be recouping those funds."
Commonwealth says because of maintenance issues it took its ATM, online banking system and Eftpos offline.
Police and the bank say it is not yet known how much extra money was dispensed.