We aren't taking our carseats as the airlines are hard on them but also now it comes out of the childs luggage allowance so only leaves them with one bag for 2weeks worth of clothes and shopping. This is with Air NZ though and I checked with Qantas and they confirmed the same thing. Not sure what V-Australia would be like.
You don't need the carseat on the coach but do on the shuttle.
We are doing a rental car for 2 days and the carseats from them are $13 a day per child.