Unfortunately, it also means it is expensive. The main carrier is Air Niugini, with several small operators touching down at very small airstrips.
Bookings are quite reliable as most systems are computerised.
If travelling on any of Air Niugini's domestic flights, make sure you check-in an hour in advance, otherwise booked seats will be given to passengers on waiting lists. Essentially modified Japanese minibuses, PMVs are a cheap form of transport and pick up and drop off people at any point along a pre-established route. Some authorities suggest that if you are involved in an accident, keep driving and report the incident at the nearest police station. Costs are also high. A good form of transport is boat.