i'm using digicel pre-paid. so far so good but a bit costly. my friends are saying that a postpaid account (mi-fi or router) is better. 250K per month for 2GB.
I am not from PNG but I have been there many times. Internet is very expensive because the company itself has to buy the connection either from Singapore or Australia. The most famous one is called DIGICEL. You can inquire if how much.