I flew (or tried to fly) Air Niugini for a trip Melbourne-Brisbane-Port Moresby-Kavieng and return Easter this year. Plane cancellations and delays by them meant I took an earlier Qantas flight to Cairns outward bound to take a Qantas-operated plane Cairns to POM instead, and on arrival discovered my early morning domestic flight to Kavieng (whole point of flying to Cairns) had been cancelled anyway - it went the next morning and I spent the night in Port Moresby.
On the homeward journey the Kavieng-POM domestic flight made an unscheduled and unexpected stop at some random airport to pick up passengers they abandoned the day before, and as a result I missed my flight out of Port Moresby to Australia and spent another 8 hours in POM airport watching locals put their guns through the scanner and pick them up on the other side. Qantas rejigged my Brisbane to Melbourne flight to get me home that night.
In summary, I recommend you have some extra days in your schedule. Through all of this I carried a backpack (camera, lenses, housing) and a briefcase pelican case (strobes, batteries, cables, chargers), weighing about 12kg each. I think the most dangerous part of this journey for hand luggage was Brisbane Airport who are known to be tough on hand luggage, but in PNG they don't care much. Noone asked to weigh either bag although my arms were a lot longer after carrying them around airports for three days outward bound and an 18 hour homeward bound trip.
On a travel agent's recommendation I also joined Club Niugini, or whatever it's called, which is free. Their website advertises that this gets you 30kg of checked luggage instead of 20kg. I gather they're trying to get people onto their frequent flyer program rather than just using Qantas's. Again, noone asked me about it or questioned my approximately 24kg checked bag.
Good luck!
