Hongkong airport sleeping capsule?

Do you have an onward flight? If so, go straight through transit to the departures level. If you are travelling Business, head to the Pier lounge as the above post suggests. Failing that, head to "relaxation corner" near gate 23 which has lounger chairs for sleeping (https://www.hongkongairport.com/eng/passenger/departure/all/airport-services-facilities/relaxation_corner.html). There are pay-in lounges with showers and plenty of coffee shops, which can help overcome a lack of sleep.

If you don't have an onward flight, you'll be stuck in the arrivals level and there's nothing there but a few seats, so you're better off going through immigration straight away. Outside in the main hall, there's plenty of seating. There are a few wooden benches at each end of the hall but if you lay down on them someone might ask you to move on. There are plenty of restaurants but they won't be open until 6am. If you are travelling Business, you can go to the Cathay arrivals lounge and get a meal and a shower (again it doesn't open until 6am), but again there's nowhere to sleep. Best option to sleep is the Regal Hotel which is attached to the airport. They are used to odd check-in times as they cater to flight crews, but as far as I know they don't charge on an hourly basis, they charge for the full night. 

That Sydney flight is a pain, it gets in before most of the airport is open, even the Airport Express train doesn't run that early. I got it once but at the time I was living in HK so I could just go home and sleep.