How to book Cathay Pacific flights using Qantas points?

MEL-HKG is easy enough, but how about say MEL-BKK via HKG? While both flight available separately they unavailable when you try MEL-BKK. Sure you can book two separate flight, but it means that you have to spend more points and potentially have arguments about connection. Booking "chain" (or "stop-over") flight is pretty much the same as two separate booking and even worse - if you need cancel just one leg you have to have long conversation with customer service. Another example is MEL-HKG-LHR - QF site only selling CX in economy while both legs available in CX business separatelly.
So how to book CX to other destination via HKG if it unavailable as "straight" booking while both legs available separately?

Call QF. Being platinum or above is a massive bonus with reward bookings but try your luck even if not. I've not noticed any issues booking the whole journey with CX. 

Had the similar issues with booking BNE-BCN, could get the flights as individual sectors BNE-SIN on QF and then SIN-DXB & DXB-BCN on EK but even then when I tried to finalize the booking it would always have an error message eventually managed to book it with an operator for the same points and $ as I was quoted online. Try booking it as a multi city flight If you manage to keep the stopovers under 24 hours in each city it works the points out based on the total combined distance.

Ah it sounds like you have a case of the 'married segment' problem I also encountered on CX... As I discovered booking my OW J award with QF points, CX block award availability for certain city pairs to/ex Australia... in my case I found availability on the individual segments CMB-HKG and HKG-MEL, however when I tried to book over the phone with QF and it kept showing up no availability, after quite a number of conversations an agent eventually explained this is the case on a whole range of city pairs (I'm not sure if they are doing this for Y as well)... The knowledgable agent I spoke to said that even if you manage to book on one of these city pairs (e.g. via QF call centre) CX will auto cancel it. The only way to get around it was incorporating a stopover (>24hrs) in HKG... Sorry I don't have a better solution, just wanted to add this to hopefully save you a few hours in discovering that 'hidden' rule!!