Actually the old CAP 509 syllabus was much easier! There were less subjects and a lot lot lot less questions for the examiners to choose from.
The Oz ATPL theory will help a little, but its a walk in the park compared to the JAA system!
I have taught the CASA ATPL, the UK CAA ATPL and the JAA ATPL theory for the last 15 years. The old UK system ( still in use in parts of the world like Malaysia!) was great as we knew all the questions that were possible, you could learn it in 4 weeks by simply teaching the questions and a bit of knowledge. The current system is a nightmare, and they constantly change the syllabus and the questions!
Good luck, recommend OAT or more so Bristol GS for a course if you want to undertake this route. Do the pre exam brushup in Perth and the exams there , but make sure you can pass the medical first, not much harder than the initial Oz Class 1, but they are picky little buggers over there!