As I mentioned in my post, there are ways to force Steam to download from Australian content providers who are affiliated with the Australian providers, one way of which is to use the program(s) linked to in that thread. Whether or not that will improve your speed, I'm not sure. As Zips said, it's an ISP issue with some, or for all I know, all, Australian providers capping downloads from Steam servers. You'll have to do some investigating on the subject, it's worth asking your ISP and checking out the Whirlpool forums for example. You may find that it is the reverse of what I am suggesting, that you will have to block the Aussie download servers in order to force it to download from overseas, and get the speed that your connection should have.
To the best of my knowledge there won't be any opportunity to download the game or updates outside of Steam, so if you find that your ISP is inflexible on the matter, or that there isn't a way around it, it looks like you'll either have to take a pass, or wait a bit longer for the initial download (unless you get the DVD) and any updates.