If you fly to MEL, there may be less Diamonds, but at the same time the J-cabin might not be full to start with, hence no upgrade. It's not uncommon for HKG-SYD routes to be full in J whilst 80% capacity in Y (word of mouth from CAs). I don't know how many diamonds would fly HKG-SYD (compared to say HKG-LHR), but definitely the quality of pax is not THAT high...

But, now that SYD has 3x daily, it may be less chance for full cabin. But the + is that you have flexibility. Can't say for MEL, but you can virtually and depart as quick as in 2 hours, or can arrive 6am (CX111) and depart 10:30pm (CX138) giving you both time to sleep on plane, and day tour in SYD.

I think CX111 - CX138 are most popular amongst J-pax, because they have minimal chargable hours to be wasted on these flights.