PNG varies in places you can go, cultures to see, flora and fauna and levels of safety you come along.
Port Moresby is unexciting, dirty, expensive to stay and dodgy to move around yourself without a guide. Transit place only for me.
Milne Bay Province is - on the other hand - very safe place to travel. Na rascals to speak of. As Alotau is not connected with Port Moresby via highway, the trouble stays in the western parts of PNG - highlands and Port Moresby itself.
I have heard of some rascal activity in the New Britain. I would suggest to check current situation in the country before you go but I would not be worried unless there is a major unrest happening. And once you are in PNG, use common sense - do not go out very late by yourself, go with locals they know where it is ok to go and where not - in Milne Bay Province you can go where ever you would like to - pretty much anytime.
Speaking to trustworthy locals is the way to go, they will help you with pretty much anything, from accommodation to things to do.
Do not believe the negative hype and experience yourself, it is worth it and you will be just fine.
