NorthOfNorth wrote on Apr 23
rd, 2014 at 7:32am:
Also there's another problem with risking 'no duty to act' (if in fact you don't have a duty to act in the first instance) where a bank error in your favour (under the circumstances of the OP) has occurred.
That is, if, say, an unexpected payment (say rates) incidentally uses some of the one million dollar balance. While you may not be aware of it at the time and you know that your pay (or a legitimate payment will be made to your account to cover the rates payment within a day or two), you have in fact unlawfully used the money originally transferred to you in error. As you have not informed the bank of the error, you have little defense against any charge of theft subsequently brought against you. Insisting that you had effectively paid it back almost immediately will not be a defence that will exonerate you from the original charge.
I think that one's borderline.
There is still a question of whether there was any intent to deprive and any assumption of ownership rights.
The theft laws (at least in Victoria) have been updated to incorporate electronic theft, but these require the person getting a machine to do something they shouldn't, which I don't think would apply in this instance.
The issue of whether you knew what was happening is also relevant. Cods' example of a person being in hospital and not even being aware of what was happening in their bank account would make it extremely difficult for any charge of theft.
The duty to act relates to the issue of taking reasonable steps to return something to the owner. However, just because I find something does not oblige me to do that. It is only if I assume ownership rights that I must take reasonable steps to find the owner.