The Commonwealth Bank could not reveal today how much money was overdrawn from accounts yesterday using the bank's ATMs.

Neither could it say how much has been returned since a police warning that keeping cash that's not yours is a crime.

A technical problem meant all the bank's ATMs were placed into offline mode yesterday. Customers could not access their account balances but could still withdraw money, even more than their accounts held.

As word spread, especially via Twitter, money was taken from more than 40 malfunctioning ATMs and police issued a warning to people not to exploit the situation and take money that was not theirs.

Two young men have since been charged with dishonestly obtaining financial advantage by deception and police warn that more charges will follow if those who took money do not return it.