Simon, for small items Aussie Post would be the simplest and most convenient way to go - I've managed to ship things as big as a 4AG manifold and exhaust headers through their services.
For larger parts like motors, I know JMS uses Northline Freight Management between Adelaide & Brisbane - their prices are quite competitive but I only have the Brisbane number up here. I can try to arrange things up here if you can't find a number for them in Adelaide ...