TWO young men have been charged over the Commonwealth Bank ATM debacle that allowed people to withdraw more money than was in their accounts.

Hundreds of people at dozens of CommBank branches around Sydney and Melbourne queued up to 30-deep after social media sites went haywire with reports the bank's ATMs were giving out "free" money.

Customers with a few dollars in their accounts were suddenly able to withdraw hundreds and, as they texted and Tweeted their unexpected windfalls, the crowds quickly grew.

"People were running past me screaming 'Free money! Free money!'," Punchbowl Pharmacy manager Feriale Zakhia said of the people using a nearby ATM.

"Everyone was so happy. They were running around with huge smiles."

Witnesses said a group of about 30 people outside the bank's branch in Sydney turned nasty when trying to cash in. "People were yelling at the top of their lungs that all this money was spilling out of the machines," witness Peter Kastarian said.

"All these guys were jumping on top of each other to get to the front. It was insane."

But, like most free things, it was too good to be true.

Police soon warned people caught withdrawing more than they owned would be traced and could face up to 10 years jail for fraud.

Police said this morning that they arrested two young men after observing a group standing outside a bank at Merrylands, in Sydney's west, about 2pm (AEDT) yesterday.

They were seen exchanging amounts of cash before three of them left in a car, which police stopped a short time later.

Two of the men were allegedly found in possession of an amount of cash obtained by using a faulty ATM.

A 20-year-old man was charged with dishonestly obtaining financial advantage by deception.

He was refused bail to appear at Fairfield Local Court tomorrow.

An 18-year-old was charged with the same offence and will face Fairfield Local Court on March 23.

Police would not confirm the incident related to yesterday's technical glitch that plagued ATMs operated by the Commonwealth Bank.

The problem forced the bank to put all of their ATM machines into offline mode.

Customers had no access to their account balance but were still able to withdraw money - more than their accounts held.

The technical glitch was repaired by about 5.30pm (AEDT) yesterday.

NSW Police fraud squad commander Col Dyson told The Daily Telegraph that ATM frenzy appeared to be the largest and most widespread incident of its type in his 32 years on the job.

"It's the first I've heard of something like this happening on such a scale," he said.

"If these people know they're not entitled to the money then certainly they are committing an offence."

But he said many people were probably more focused on the dash for cash than whether it was legal. "I think when they are presented with an opportunity like this a lot of people act without thinking and then repent at leisure once they think about their actions and the possible consequences," Supt Dyson said.

Bank spokesman Bryan Fitzgerald said the trouble began when the bank attempted to perform a maintenance update on its computer system overnight. By noon, it meant people could withdraw any amount up to their daily limit, regardless of how much money they had in the bank.

Those withdrawal limits are up to $2000 a day for holders of keycards and debit Mastercards.

"No one has received free cash," Mr Fitzgerald said.

"What they've done is overdrawn their accounts. We will be following those people up and recovering that money."