I don't want to rain on your parade but I'm certain you'll have to collect your bags in ADL after the QF flight due to an overnight break of journey & QF don't store bags overnight.
Even if all the flights are in the same pnr when you checkin at MEL the CSA will only see the MEL/ADL flight as the CX flight ADL/HKG is the next day. It's not always the 24 hour rule if there's an overnight break of journey in Australia you need to pick up your bags that night then checkin again the next morning.
The same thing currently happens for evening departures BNE/SYD with onward connections the next day eg BNE QF x/SYD OZ SEL or BNE QF x/SYD AC YVR. For the purposes of ticketing on an international journey the IATA definition of a stopover is anything over 24 hours however whether your baggage can be checked through is not solely dependent on that rule.
Is it really a big deal if you need to collect your bags in ADL & checkin the next day?
Yeh well is sort of is a big deal because with no bags you could head straight to wherever your headed (a meeting, dinner, coffee, whatever) as opposed to waiting for luggage then having to drop it off somewhere first then having to lug it to the airport again in the early hours of the next morning.
I thought having luggage checked all the way through was one of the benefits of having it all on the same PNR and I thought this was especially likely as CX and QF are in the same alliance.
How disappointing looks like QF have let me down again lol are they the only airline to have this rule of not storing bags overnight ? Probably wouldn't surprise me they're probably the only airline to do a few things that adversely affect their customers for example offering a cafe breakfast in international J lol.