Usually, if all the other houses in your street have their cable entering their properties via overhead cable, then the standard install would be overhead cable.
I would think that if your premises is serviced by underground conduit and it has an existing opening at the pole in question, then I don't see that it should be an issue to get the cable installed through the existing conduit (assuming that the conduit was sized to accomodate the cable plus the copper line).
You either need to call the Sales team and be very specific with your requirements (they can not confirm the install method themselves) or preferably get on Live Chat and save a copy of the conversation for future reference.
Alternatively (and possibly the best depending on the quality of the staff), pop in to your nearest Telstra Store as they may have had experience with this situation before in your area.