What you'll find is that the Australian ATPL papers concentrate on turboprops and bigger. No piston stuff and only a little bit on props.
Remember how much stuff you had to learn for the JAA? Well, instead of it being "spread out" per se, it's a lot deeper. The toughest paper for me was flight planning. You having flown a "heavy" should be able to make a lot more sense of it all which is useful.
They don't take 6 months to do either! There are a few theory guys around that can get you through them fairly quickly.
I would do the CPL bridging paper and the IREX. That'll "get you working" as such. Then knock off the 4 ATPL papers. As you have a JAA licence, it may not apply but I know Virgin Blue were not accepting CASA ATPLs based on an ATPL bridging exam. (Folks would head to the US and get the FAA ATPL theory done and then head back to Australia with it, thus avoiding the dreaded CASA ATPL theory)