Probably best to think of your trip as 2 x one way journeys each of which can have one stopover
so today
LHR - BKK - SYD (one way LHR - SYD with stopover in BKK)
MEL - SIN - NRT - LHR (one way MEL - LHR with stopover NRT)
you want to leave the outbound as it is but you are looking to change the return to include 2 stopovers, as only 1 stopover is permitted this will not be allowed on a single trip (you are now wanting one way MEL - LHR with stopover AKL and stopover NRT)
To get this will not be impossible but you will have to break your return into 2 journeys so your Z9 - Z1 becomes either
(MEL - AKL) + (AKL - LHR) ... an additional 22500miles + cash/taxes
(MEL - NRT) + (NRT - LHR) ... an additional 15000miles + cash/taxes
I think/hope that makes sense