I reckon the choice for lounging is between the Qantas Lounge and Cathay's The Pier Business. They're at either ends of HKG: Qantas at the 'top' (near gates 5-6) and The Pier around gate 65. Both lounges are quite different and both have a lot going for them, although the best Qantas lounge food rolls out in the late arvo ahead of the QF flights. I really like The Pier for its noodle bar and tea room. If you have a short transit then which lounge you visit will really depend mainly on which end of the airport your HKG-SYD flight departs from.