I did the FAA ATP conversion last month.
As advised from the above posts, the easiest way is for you to convert from OZ ATPL to FAA PPL based on your Australian licence. It's the highest licence the FAA will issue. FAA validation took a week to process. It took me one day to study the ATP written and passed >90%. Once you have the ATP written, organise a checkride SEL or MEL through the local flying school or Type Rating with a FAR Part 142 training school.
Good luck!
