I suggest you email your question to CASA Flight Crew Licencing, any response you get from here will be best guess and possibly based on outdated info. CASA have a task force working on correcting a long list of "unintended consequences" as the result of the introduction of CASR Part 61, ATPL overseas conversion is on the list.
Try:
[email protected]
Then you could go to the airline with the CASA direction, the airlines will have no idea of what's required to convert a foreign ATPL. Most Australian carriers will want at least CPL, IR-MEA and valid ATPL subjects as a minimum or current Australian ATPL.