If you are hoping to convert a US ATP to an Australian ATPL, to add to dr pixie's and awair's posts above. If you already hold an Australian CPL it is not the easy path it may have been many years ago. Even for those who are initial applicants for an Aussie licence by converting a foreign licence, CASA looks hard at whether the applicant has actually operated on the particular licence, or is simply trying to circumvent their 'system' by rocking up with something bought last week out of a vending machine in Mumbai or Memphis.
As far as I am aware, there are no reciprocal arrangements between CASA and the FAA for equal licence transfer. Both may issue one grade lower - ie an ATP in one country gets you a CPL (with not much more than a medical and air law required) in the other.