I obtained my naltrexone prescription from my primary care doctor, but many people who are doing The Sinclair Method (TSM) get their prescription online here (http://www.blueriverpharmacy.com/). It is expensive online, but it sounds like this may be your only choice. I highly recommend checking out the TSM forum: http://www.thesinclairmethod.net/community/ All members are people at various stages of the TSM process from all over the world and you may be able to find a doctor in Australia in your area. The very least, you will get lots of advice and information from this site.
Naltrexone is pretty great, but lifestyle modification is also key which I've learned after a year of taking naltrexone. My old routine of sitting on the sofa every night and drinking had to stop, so now I do other things in the evening and it helps.
I also read recently that extremely low doses of lithium reduce alcohol cravings. I just read this in a book called "Breakthrough" by Suzanne Somers. She interviewed top doctors about various health issues and alcoholism was one of them. I may even try this in addition to naltrexone. Also, an herb derived from a weed called Kudzu has been used in China for many, many years to treat alcohol cravings. Harvard did a study on it and it showed reduced drinking. It intensifies the effects of alcohol, so the idea is you need less alcohol to feel the effects.
Good luck, I hope some of this might help. Please feel free to ask me any other questions. I will email you my private email, so you can reach me there.