Up to 40 of the Commonwealth Bank's ATMs were giving out more money than people had in their accounts. When word got out, people lined up to get extra cash.
"A young bloke at the machine had put the card in and just spitting out lots of five hundred dollars," said one man.
For other customers who wanted to withdraw money, they found their accounts had been emptied.
The bank had to shut down. Its entire ATM network and online banking also had problems.
Officials warned the public that those who took money that they were not entitled to could face a serious criminal offense.
The meltdown was blamed on maintenance work carried out the night before.
It's second major glitch for the Commonwealth Bank in three months.