The travel agent has got back to me about the continent flight limit and he has confirmed that I was correct.
He has also advised that as a QF ticket taxes will be about AU$1100 and as an AA ticket taxes will be about AU$800. Does this sound about right? Is it because of all my transits through LHR that is making the taxes so high? I tried making my itinerary go through MAD or HEL, but couldn't get any availability with decent connecting times with IB or AY.