If you do the ATP before going to Aus you will only have to do the CPL airlaw and the ATP airlaw with no flight test. Although your IR is a defferent ball game. You will have to do the Irex and a flight test in a IFR machine. Most people just do the Irex and wait until their employed and usually the employer will do the flight test.
If you are going the other way from CASA to FAA. If you have a CPL and a CASA IR then forget the CPL conversion, go straight for the FAA ATP. You will have to do the same thing for either. An ATP theory exam and a flight test. You do not need a instructor endorcement to sit the ATP flight test. Although a few hours with an instructor would be very wise.
DT