It has been a few years since i was up there but i recall avgas available at Madang and it was the nicest and most relaxed place that i stayed. Beautiful spot. My grandad was there in WW II with the RAAF and loved it there though he hated milne bay whilst he was there.
According to AC-U-KWIK AVGAS and JET both available at Madang. I do not have a current subscription so cannot access phone number sorry. I recall the local GA operator was very helpful (john song or so?) not sure if he even still there.
If you do not have complete WAC coverage (paper) i recall that the big old Readers Digest Atlas of Australia has old Wac for PNG. I know ONC were nearly impossible to obtain not so long ago.
HD