Licences can only be converted "like for like". So if you wanted to convert right now, you could only get a JAA/EASA CPL, not ATPL as you don't hold an ATPL.
As Australia is not in the European Union, then whilst it's ICAO compliant, you'd have to convert it to operate an aircraft registered in Europe/JAA/EASA land.
JAA ATPL theory was much broader than the Australian one, mainly because there's no real CPL syllabus. The Australian theory goes a lot deeper. Both require remember a tonne of stuff you'll never use. There are a lot more exams (and cost) with respect to the JAA theory.
Some regulatory authorities accept Australian ATPL theory passes with Air Law usually being the one you've got to do. I don't know which ones though.
The only Australian person I know who passed the JAA ATPL theory exams with "ease" had over 7000 hours, 4000 hours on Bae 146, 500 hours on the B737 and used to teach PPL,CPL and ATPL theory. They're not impossible, but they do require some effort.
An Australian licence can be converted. It is dependent on how deep your pockets are and how willing you are to go through it.
I did it 10 years ago.