I am currently in Sydney and leaving in two weeks. My final destination is Atlanta.

I have 3 weeks to travel:

I wish to go to:
1. Syd - Great Barrier Reef (not sure airport Cairnes/Brisbane or something else)
2. Great Barrier Reef to New Zealand (probably Auckland)
3a. Auckland to Bangkok (might be easier to fly back to Sydney first?)
3b. I also want to visit Cambodia (thinking easiest to do Air Asia while in Thailand and fly back). I would prefer to go first to Cambodia if flights made sense
4. Thailand back to the US (final destinatio Atlanta)

I'd like to spend about
3 days in Great Barrier Reef
5 days in NZ
7 days in Thailand
5 days in Cambodia

Any tips on booking flights as far as combining etc?