What is the cheapest airline flying fom qld. australia to cambodia?

You're not going to find anything cheaper than Air Asia. Better? Yes. Cheaper? No. Better airlines would include Cathay Pacific/Dragonair through Hong Kong, Singapore Airline/Silk Air through Singpaore and Thai Airways through Bangkok, but all will be considerably more expensive than Air Asia. Jetstar through Singpaore should be competitively priced with Air Asia, and has about the same level of service.