Adelaide is 1.5 hours ahead of Hong Kong
Best overlap: 8:00 AM to 5:00 PM in Hong Kong is 9:30 AM to 6:30 PM in Adelaide.
Shoot for 8:00 AM to 5:00 PM in Hong Kong; your Adelaide colleagues will see that as 9:30 AM to 6:30 PM. That's a roomy 9 hours of common time each day, so finding a slot that suits everyone is rarely a problem. For part of the year the maths changes: Adelaide observes daylight saving, Hong Kong does not, shifting the overlap by an hour.