Well before 1974, has to be prior to the formation of Air Niugini in 1972. My guess the second last photo of the Navajo, Skyvan and Patair DC3 at Port Moresby could be the late 1960s?
The C336 is VH-GKY I suspect. I think that is the aircraft Fris has at the Queensland Air Museum?
My guess: first photo Madang; second photo unknown; third photo Goroka; fourth photo Port Moresby; fifth photo unknown.
The Lear 24D registration VH-SBC has an interesting history and PNG connection. It was originally carried by a TAA DC3 (C47A) in PNG and is now carried by a Kavanagh hot air balloon. The Lear 24D belonged to the Swan Brewery Company up to the early 1970s. I have seen that Lear in Port Moresby in the late 1960s when Swan and San Miguel were in partnership and built the brewery at Boroko or Gordon's.