To go from the CASA license to the JAA one is expensive and time consuming. The other way around is comparatively easy.
Note, you only covert licenses. If you have a CPL with IR and ATPL subjects valid for JAA land, if you come to Australia you'll convert the license only, you'll have to do all the Australian ATPL subjects and they're not the cake walk some folks like to make out. (Then again, they're a lot more "sensible" than the JAA ones)
If you're considering flying in Australia you'll have a ball and do the sort of flying that the Europeans only dream about. (or have nightmares..)
For once, the job prospects in Australia are good, even for low houred guys. Admittedly, you'll have to head away from the nice cities but that's part of the fun.
I've got both JAA and CASA licenses.