We go to HK in 9 weeks, and sometimes it suddenly hits me, what we're doing - dragging three kids, two of whom are teenagers, into the unknown, giving up comforts, friends etc. But what an adventure!
I think you need to think about it logically. There is tons of stuff to do and see in HK, thousands of expats in a situation similar to yourself - I'm sure you'll make some good friends soon enough.
Something that reassures me is reading the posts on geo expat. The people who live there love it, by and large. They seem a very friendly bunch! Don't forget that with skype and msn, you can keep in very close contact with your Sydney friends and family.
I've decided that when we get there, the onus will be on us to have a good time and enjoy it. Decide it's going to be a fantastic adventure. Decide to get out there and meet people and settle in fast.
I'm sure you'll do very well!