you'll be in the Hong Kong winter, and the temperature can vary between the low 50's and the low 60's Fahrenheit. I've never found it necessary to wear a sweater there at that time of the year, but my wife sometimes does. It will feel good, because the humidity drops considerably.
As far as Australia goes, it depends on where you'll be. I mainly go to Sydney, and in December the summer will be getting under way. It's normally around 75 to 80 Fahrenheit, but can drop to the low sixties. You'll probably see some rainfall, but not too much.
My favourite airline for the Hong Kong/Sydney run is Cathay Pacific, but I'm sure you can get better deals than they'll offer click here