Stevieboy, I think you have a typo. You mean convert from Oz ATPL to FAA *ATPL* don't you? And not FAA PPL as you wrote?
If you did mean PPL there's no need to get the FAA PPL. Go straight to the ATP flight test and be done with it. Also any ICAO CPL allows you to do the FAA ATP flight test and not only ICAO ATPLs.
Something I should have mentioned: Get the Practical Test Standard for the ATP before you start. The PTS lists everything that you have to do on the flight test as well as 'may do' stuff.
For the theory Gleim, ASA & Jeppesen each publish a quite useable textsto prepare for the computer based multi-choice exam. They each have slight differences in how they do things but I liked the variety as a way to emphasise the information. You don't need long to study the stuff. A few days to a week, at most, if you get stuck in. I used Gleim & ASA and studied on a flight to the UK, a few nights in the UK, the subsequent flight to the US and then a day or two in the US.