If doing an AA Platinum Challenge (aka Candy Theft), you will complete the challenge on the HKG-DXB segment if flying in business class. You will earn the following AA miles for the segments (assuming starting in SYD):
SYD-HKG = 4581 miles flown, 6871 EQP, and 5726 AA miles
HKG-DXB = 3684 miles flown, 5526 EQP, and 8289 AA miles
DXB-HKG = 3684 miles flown, 5526 EQP, and 8289 AA miles
HKG-SYD = 4581 miles flown, 6871 EQP, and 10307 AA miles
Totals = 16, 530 miles flown, 24,794 EQP, and 32,611 AA miles.
This is half way to requalifying AA platinum in its own right for the following year, will earn you 4 x 500 miles AA domestic upgrade credits, and is almost enough AA miles for a business class return trip anywhere in Australia/NZ without paying the fuel surcharge.
The benefit of undertaking the AA challenge depends on your future travel patterns. But just for this one-off trip, plus crediting a few hotel stays, rental car or similar partner miles to your account, you are very close to earning a free trip, while if crediting to your QF account you will not get close to such an award.
Note that EK is going to be very cost competitive for a business class fare to DXB.