Download quotas have become more generous since Steam, Valve's digital distribution client for games and software, was released. That said, if you can force it to use servers that are unmetered for your ISP without compromising performance, there's little reason to waste bandwidth.
Rather than tweak your firewall settings or dig into a hosts file, you can just install Nigel Bree's steam-limiter. It comes pre-packaged with profiles for Telstra BigPond, iiNet, Internode, Optus, iPrimus and Adam, which should cover a good chunk of Australian gamers.
Once installed, the program is required to stay active while Steam is running. It will then prevent Steam from connecting to non-approved servers... and keep your quota happy.
There are some limitations with the tool, though these shouldn't affect the vast majority. One thing to keep in mind is that you should start steam-limiter before running the Steam client, to make sure it works correctly.
steam-limiter [Google Project Hosting]