I've been told that the computer picks the end-point, and does so to make the open-jaw legs as similar in length as possible. For this example it will become SYD-LHR + LHR-AKL, and there is no leeway for the agent to change it. This game is only profitable if you have a need to visit a 3rd city for a few hours (transit instead of stopover), and save the mileage for routing to it (and the airline schedules allow a transit). I think the intent behind not counting the mileage to intermediate stops is to avoid penalizing you for the routing that an airline happens to implement (and which can change over time). This makes the plan fairer and more stable over time, without opening up significant loopholes for abuse (not that we'd be discussing such a topic here).