The media have been revelling in the search for Westpac NZ customers who were mistakenly paid NZ$10M instead of $100,000 and allegedly promptly took most of it and disappeared (for example, ABC News).
Whilst the media have accepted the obvious point (that it was a mistake and not the customers' money) they have also focussed heavily on the Westpac employee who processed the transaction.
Ozrisk makes the point that Westpac's systems have failed in firstly permitting the transfer of the wrong amount to the customer and secondly allowing the customer to transfer the funds to other accounts.
The law on mistaken payments is clear: bank customers are not entitled to keep money deposited to their account as a result of an obvious mistake. The customer who receives the money is obliged to return the payment unless, in good faith, they have changed their position to their detriment. They would only be acting in good faith if they genuinely thought they were entitled to the money. They can't treat the mistake as a windfall.