If you haven't seen this then the CASA website has a section on converting foreign licenses.
Civil Aviation Safety Authority - Converting foreign licences
Also you might want to look at RA AUS
Recreational Aviation Australia You can only fly day VFR 2 seaters and you cant fly in controlled airspace unless you have a PPL as well but you probably find it cheaper and it depends what type of flying you are wanting to do. Some of the top end machines will do 130kts and members tour round Australia quite happily. You can also get an ASIC through them I think.
I have found both organisations very helpful whenever I have phoned them.
I have an Australian PPL and am a member of RA Aus so do both.
The main GA field at Brisbane is Archerfield but I havent been there for ages so have no idea what it is like now. I live in Sydney and so go to Bankstown and was lucky enough to have an Australian PPL from when I was out here previously. In fact I got my UK PPL on the strengh of my Australian one.
Hope this is of some help.