A few years ago I donated a B20 ride cymbal to a young drummer in England. That bit of goodwill cost $145 through USPS, with additional levies once it reached England. I tried every alternative, and that was the cheapest route I could find. Shame. I will not be shipping anything internationally anymore.
Get a 22" cube (will be cheaper than 24").
Nest the 14 inside the 18". Cut height of box to 18" or slightly less. Remove hardware from 12 and nest inside 14 or ship in a separate box 18 x 18x 12. Will be less than 200 to ship.
Yeah, it's highway robbery to ship internationally. I live in Costa Rica and the shipping costs to or from the US are similar.
However, for Costa Rica at least, there are mail forwarding services to receive packages from the US. The US shipper ships to the service in Miami, the service in Miami then handles the legal stuff and ships to Costa Rica. It's still not cheap, but it can be better. However, size does matter. I've gotten a few cymbals this way, but a bass drum is out of the question. I don't know about a snare.
Also, I have no idea if there are similar services for Australia.
PS. There are also mules. Find someone flying there anyway who can carry it in luggage. Sometimes mules advertise on discussion boards.