Like Mick said, its the AAA or NRMA (if you are in NSW)
Read this article re costs on the AAA site. I've been speaking to someone about geeting one in NSW, and the woman reckons about 2-3 weeks to issue.
Taking Your Car Overseas - Australian Automobile Association
It is valid for 12 months, but you can extend it while on the road. The option is to extend for 3 months and then again for a final 12 months, or to extend for 12 months the first time.
The only thing I'm not sure about yet, is how it will work with the road tax (rego). From my understanding, the carnet is only valid if your car is registered here in Oz, which means you need to have paid your rego for the year. This seems pretty stupid though seeing that you won't (in my case) be using the car in Oz while the rego is valid. this also means (in NSW) that you have to have the green slip insurance. All of this is spending money on nothing basically.
I'm not sure if I misunderstood the person I spoke to originally. I will still speak to someone at the RTA about it though.
Perhaps someone has gone through the process recently who will be able to give more accurate info re this...