Welcome to FT.
Never had any great luck trying to book a 'holiday' return for 1 or 2 pax LHR-MEL with quite fixed dates. However my son was over in the UK last month and needed a one way ticket LHR-MEL. I was able to sort out lots of options with CX at short noticeand at one point was holding 3 Avios tickets on separate dates - one in J via Frankfurt and HKG and 2 F to HKG and J to MEL. A lot of award seats opened up at short notice on CX and just for fun I was able to find quite a few returns back from MEL but only within a 10 - 14 day window.
Looking at March next year BA gives no availability but if you include partners I find one or 2 days with CX via HKG in J. Tons of outbound availability on 1st April only ??why
Otherwise maybe book a reward to HKG with BA or CX and book a second reward or pay CX or QF for the HKG - MEL sector?