I converted a JAA ATPL to the aussie ATPL. I don't know where you are getting the idea that you need a CPL as well. To convert to an aussie ATPL you do have to complete the aussie CPL law ground exam, just to give you a good grounding in the subject before you complete the ATPL law exam.
You might want to check you facts. If you have a JAA ATPL you will be treated in Oz the same as everyone else with an ATPL regardless of what route you took to get this licence.