The Kumuls and Kiwis will clash in Parramatta on Sunday in a promotion/relegation showdown, with the victors assured of a slot in next year's top-tier Pacific Cup.
PNG led 16-14 at halftime in Port Moresby against the Aitu after an underwhelming half but went on with the job.
Skipper Lachlan Lam and fullback Nene Macdonald, who scored two tries, starred, with both conjuring up decisive plays when Cook Islands took a 14-4 lead.
PNG captain Lachlan Lam scored a try in the Kumuls' win over Cook Islands. (Supplied/AAP PHOTOS)
The Kumuls opened the scoring through winger Elijah Roltinga, but the visitors stunned the home crowd, with veteran Alex Glenn sliding in to dummy-half and putting barnstorming prop Davvy Moale in under the sticks.
The South Sydney front-rower copped accidental knees in the ribs while scoring and was forced off the field.
PNG forward Sylvester Namo was binned for a high shot on Nua Ngatikaura.
Aitu winger Steven Marsters scored seconds later and the crowd was silent, with the Kumuls falling 10 points behind.
Two moments of individual brilliance turned the match with Namo in the bin.
PNG fullback Macdonald carried five defenders over the line with a blockbusting run.
Lam, captaining the Kumuls for the first time, put on a left-foot step 35m out and accelerated away to score. He pointed towards his legendary father Adrian Lam in the stands and the crowd went wild.
Macdonald raced 40m after halftime before a floating Lam pass sent Robert Derby in. From there the floodgates opened, with the Kumuls scoring four tries in 15 minutes to put the game beyond doubt.
The Kiwis will be favourites next week to beat the Kumuls. PNG's one Test win over New Zealand was in 1986 when they prevailed 24-22 in a World Cup fixture in Port Moresby.
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