Make sure that you front with a current CPL and current medical and current instrument rating. They can only validate stuff which is current. Not current? Back to where you came from to get it done. By ICAO rules the validation should be good for 90 days, but in PNG they need the licencing revenue, so your fee will only buy you 60 days. That's plenty enough time to go to work and sit PNG CPL Air Law and the I.R. Air Law and do a local medical. You also need a letter of verification from your foreign CAA to tell them that the licence with your photo and fingerprints all over it is not a forgery. Then they will issue a PNG CPL with whatever endorsements are in your (still current) foreign licence, provided that those types are on the PNG register. To my knowledge they don't have their own ATPL theory exams, so once you have all the above, you can then hit them with the NZ ATPL subjects, sit the ATPL Air Law, pay more fees and provided you have the requisite hours, do a flight test. Try to short cut the system and it will end in tears - for you that is.
Or you could just front with a foreign ATPL in the first place. Then you only need to do the two air law exams and medical and bring along the letter of verification.