YT, that's exactly right. A 'double open jaw*' AMS-???-MEL; SYD-???-UK should also be possible without being subject to UK APD. Try it on one of the airlines web sites. But do some comparisons, returning to the UK rather than AMS/PAR could have a higher fare just because UK fares are higher.
APD applies to flights departing the UK and this itinerary doesn't have any. But do make sure that your initial UK-AMS/PAR journey is ticketed separately. You do pay £13 (economy) or £26 (business) APD on that sector if you fly.
*As I recall the terminal points of the open jaw must be in the same country, with the EU being considered one country for this purpose. Don't delay your trip too long!
Last edited by ExXB; 11th Feb 2017 at 16:57.