See post #2 for a complete description of the process. All bags must be claimed upon international arrival and presented for customs and quarantine inspection. Then you go to the QF transfer desk to drop off bags for a QF domestic flight (very useful as at SYD you have to change terminals, so not having to lug the bags to the other terminal is nice). You can drop off bags for a connection even if they haven't been tagged through, so it doesn't matter at HKG whether your bags are tagged for SYD or for AYQ. They will be retagged at SYD after customs.