Tell the check-in agent you are continuing to sydney, and have a copy of your HKG-SYD e-ticket (or itinerary with PNR) to hand over. Your bags should be tagged priority given the connection to business class (if not done automatically, just remind them.)

This will also mean you'll get your onward boarding pass and can proceed straight to transit security in HKG (don't have to go via a transfer desk).