Go Matilda seems to be the best of them. Visa Bureau also gets some good reviews.
If you have a reasonably straightforward situation (and for Australia, 99% are because the rules are so clear cut), you don't need an immigration agent at all. Many, many people (well over half) do the whole thing without an agent and save a great deal of money. It's really not at all difficult and even the best agents can't get you a visa that you wouldn't have been granted anyway and nor can they speed the process in any way.
You will get heaps of good info at the Pomsinoz. forum which is well worth joining. It's free too, unlike an agent:
The British expats forum is also worthwhile: