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I received $1000 and was asked to send it back. How was this scam meant to work?
Most answers have concentrated on this being a scam, however, it is possible this is an innocent mistake. Australian bank account numbers do not have redundant digits to be used to validate an account number; all of the numbers are data and uniquely identify a bank and branch (the BSB number) and an account (the Account number). Computer check digits are not part of bank account numbers because bank account numbers pre-date computers. It is entirely possible that someone entering an incorrect number can, by chance, hit upon an existing account. As the bank clearance system in Australia is entirely automatic there is no cross-checking of account numbers with account names.  Internet banking in Australia is not a wire-transfer as is common in places like the USA (although these can be done): here you are effectively accessing your bank's "back office". Nor is it like the BPay service which  is used primarily by B to C businesses as a way for their customers to pay their bills; when using this service the biller code will show you who you are paying and the customer number does have check digit validation. I run a business in Australia and it has happened to us on several occasions than an employee or supplier has given us incorrect numbers. Usually, it is not a real account and after a week or so the money makes its way back to us with a message like NO ACCOUNT or A/C CLOSED.  Very occasionally, however, the wrong number hits a live account: when that happens the person who f*&ked up needs to contact their bank and try and get the transaction reversed. If there is money in the destination account this usually happens with little fuss, however, if the destination account has been closed or emptied things get problematic. Of course, taking money that isn't yours is stealing even if it happens to be sitting in your bank account. However, unless the sum involved is significant the police are usually not interested in diverting their attention away from "serious" crimes like homicide, armed robbery and terrorism so the aggrieved party is usually on their own. That said, this is probably a scam because they called you rather than your bank doing so. They cannot get your phone number from your account number: they have to know who you are and what your account number is. This is not as hard to do as it sounds since both your name and account number are prominently printed on your cheques and deposit books (possibly your phone number as well which saves them looking it up in the White Pages).