One point to bear in mind if you choose very large suitcases, is the problem with having to lift and manouvre them through airport lounges, and then get them into a boot of a car etc.
I would suggest getting the 1 size down from very large, as we find we can get about 24-25 kilos in them, which is still plenty when it comes to man-handling. If you manage to stuff more heavier items in, you will hopefully still be below the 32kgs, which is the very maximum the airlines will allow in a single case or bag. We have often seen people unpacking their cases, trying to decide what to jettison.
I would also try to book on an airline that travels direct into Adelaide, Singapore, Malaysian etc, as Quantas & BA fly into other airports such as Melbourne or Sydney, and you have to travel across the airport to another terminal to catch an internal flight.