When making bookings you may find it convenient to book on their websites where often reasonable deals can be had.

Qantas's is at:
http://www.qantas.com.au

I do not know Virgin's but an internet search using the address http://google.com.au and selecting Australian addresses should find it quickly.

I prefer to fly: Qantas; Virgin, in that order. Never considered Jet Star and probably won't unless I have to.

NOTE: when booking for international airfare to Brisbane, you MAY be able to add international legs to it to use later and it MAY not be a lot more expensive eg if flying North America to Brisbane, see if you can add a Brisbane to Sydney (or wherever) flight to it. Sometimes the international flights fly on to another city and 'domestic' passengers can later board the flight to continue eg Qantas has international flights to Darwin, that fly onto to Adelaide. It is possible to book a flight to Darwin but re-board later and fly to Adelaide. Worth seeing if it is an option and any cheaper.