I doubt CASA would allow that. Many moons ago the popular soft option to passing the Aussie ATPL theory exams was to go to the USA, put some coins in a vending machine and return with the FAA ATP.
Then CASA made it more difficult by requiring proof that the pilot had actually used the ATP in commercial operations. Like much of CASA, it tended to be whatever the FOI of the day wanted. Now the FAA licence is one of the more difficult ones because of all the security hassles and also they have upped the testing requirements.
Theoretically we are all ICAO and should recognise other ICAO State licences. But get it in writing from CASA first.
When you figure in travel and accommodation costs, going overseas to get a ticket which may not be acceptable is quite a risk. Although the PNG licence seems easy to obtain, it is a slow process because the local authority works at its own leisurely pace. Accommodation in POM is not cheap. Also, they link issue of the licence to an employer, so you will need a sponsor.
No free lunches in aviation.
While doing an ATPL in a Seminole is a joke, it is a shame that CASA don't allow someone to configure a Beech C90 for two pilot operations (checklists and SOPs airline style) and approve it for ATPL flight tests. There is enough complexity in a twin pressurised turboprop to make an effective evaluation tool.