There are two different types of insurance in Australia - the third party injury insurance which is compulsory and you can buy when you get here from the appropriate transport office. Easy to get, just have to pay the $$$s. I think the majority of overseas vehicles just go with this insurance and bear the risk of a collison which is their fault.
The other is non compulsory insurance for the damage your bike may do to others (third party property) or comprehensive insurance (3rd party plus covers damage to your own vehicle). Hard and expensive to get.
You may have to get Googling to find an Australian insurance company who will take you on. Its more likely that an insurance broker will be able to fix it up rather than going direct to an insurance company who just want to sell standard policies. There are the likes of Shannons who insure classic vehicles and bikes so they would be better able to sort something I would have thought.