Westpac is unable to verify account details for non-Westpac accounts. Should the details you have entered prove to be incorrect, in most cases the payment (or individual credit) will return to your account within 48 hours. As this return is made by the other financial institution, it does not come via Online Banking and you will not be notified. Nor will the status of the payment change. For this reason, please be careful when sending funds to third party accounts, and as a matter of course, check your transaction lists regularly for any returned credits.
Westpac cannot reverse transactions you make in error, either to Westpac or non-Westpac accounts. Should an amount sent by you in error not be returned automatically by the receiving financial institution, it may not be recoverable at all.