If you wanted to try and complete the transaction... I'd message them once, clarify that the address provided is identical to the original, which Australia Post have deemed incomplete and undeliverable, request the specific details required (eg floor / level, name of the business trading at that location, just in case they misunderstood the phrase "business name"), notify that an invoice for the postage to resend the package will be sent to the email address used for the original payment, then say that unless both payment and these details are supplied by X date, unfortunately you won't have the necessary information etc to ensure successful delivery so will have to cancel the order and issue a refund (less expenses thus far incurred).
It puts the ball in their court plus provides a time limit for a clear action to take if they don't supply what is required for successful delivery - the main problems would be is they only provided one or the other, i.e complete address but no payment, or vice versa.
If you didn't want to complete the transaction, I'd leave out the attempts to do so and just explain that unfortunately, the transaction can not be completed due to lack of information provided, and a refund (less expenses incurred) will be issued. (The reality is, it's not that different to a change of mind return, so I would essentially treat it in a similar way).
