Bear in mind that you must START your journey from where the ticket is issued. You must also pay for the ticket there. ie You cannot pay for the ticket in Australia for a ticket originating in New Zealand. Well, you can, but you will end up paying the higher Aussie rate. In order to get the lower fare, you must go there first, and then book / pay and issue your ticket.

You can however, ask Qantas or BA for a list of fares from where you intend to start your trip from. Just tell them you know that you must purchase, book and issue the ticket there. Otherwise, the sales agents will insist that they quote you the Aussie fare. Nor are they inclined to give you information on where to get the lowest fares from! After all, they have to protect their market share!