You will need to know how much the bike weighs and what its packed dimensions are .... then check with the airfreight companies - UPS, DHL, Fedex, TNT etc etc and see what they charge for freight. Ask around, coz the price varies considerably from company to company. They should also be able to quote an insured price. Checking with one of the bike importers to find out who ships bikes into the country for them might also be worthwhile.
A slightly interesting idea that might be worth considering is using Australia Post - you can post parcels up to 20kg in weight. They do have some size restrictions so it may not work out but worth checking.
The passenger airlines used to charge an arm and a leg for unaccompanied baggage, I don't know if that is still the case but that is another avenue to consider.