As far as converting from JAR to CASA its still a bit of a process but vastly easier and less expensive than going the other way!
"As a general rule, to convert a foreign licence to an Australian flight crew licence you must pass a Flight Rules and Air Law written examination and a flight test, and obtain an Australian medical certificate. If you want to convert an overseas rating (eg instrument rating or instructor rating) you need to pass a flight test and either an aural examination or written examination. Examinations and flight tests can only be conducted in Australia." CASA website
I recall to go from CASA to JAR, one has to repeat the entire ATPL ground syllabus and all the appropriate skills / flight tests... months of frustration!
Good luck with your big start though and trust it works out well for you
