I think prizes are cheaper in the Netherlands and Germany. With a stop in Singapore - Olympic Airways with 1 night in Athens. Usually a bit cheaper. You can book at your own travelagency together with your flight-ticket. Or you can directly book at an Australian car-renting company.
Some people even wait and book when they fly over.
However, you have to take the chance that your preferred vehicle is not available anymore, depending on the season you travel. I had to choose between Maui, NQ and Britz. Because of the journeys and locations of the depots, I opted for Maui. Booking was done via by agent in Europe. I had however split two periods.