An important aspect is how many night flight hours do you have? Australian pilots with 100+ night hours are few and far between (unless you fly twins), and you need 100 night hours to get an FAA ATPL rotorcraft certificate. So unless you have a 100 night hours forget about the FAA ATP certificate. And in any event, even if you have the FAA ATP you still need the required minimum hours (TT & PIC) for the CASA ATPL-H licence.