If you are looking to register an Australian top-level domain (TLD) you have many choices for a registrar and host. Your first stop should be www.whatsinaname.com.au to compare the prices between all the different registrars. Obviously you want to select a registrar and host combination (they may be different) to meet your requirements for support, up-time, cost, and features.
If you already have a host set up (it can be anywhere, not necessarily an Australian host), and you simply want to buy the .com.au TLD and redirect it, I would recommend hostess.com.au. In my experience they were fast and efficient. The signup asked whether I wanted a redirect and what the DNS info was, so there was nothing to configure, and the paperwork (including invoice) was emailed to me within minutes.