Well, I would say that it is both true and untrue that it can be dangerous in PNG. When you travel there, you must stay aware. Port Moresby has a bad gang problem with the corresponding crime. Most hotels are VERY secure. I do not travel around after dark, for the most part. Even in daylight, you need to be cautious. I have stopped at roadside vendors and had to leave when my very presence garnered unwanted attention drawing a crowd. Drivers will let you know when the crowd is a bad one and wisk you away. Listen to them.
Even in a smaller town like Kimbe, you must stay aware. Some crowds gather if you stop to bargain with vendors. They know you have money when you shop openly. Again, your drivers will let you know when it is time to go.
In the highlands, tribal warfare stems from troubles over various forms of compensation (or lack thereof). They can get lethal, but are aimed at tribal members only. I was in the Inga provence when a tribal "tiff" resulted in a man getting hacked to death. In Chimbu provence, I passed a number of burned out houses where 5 people had been killed. Tourists are not targeted for this type of trouble. Robberies, however, are not uncommon for the tourists. Hire local guides and listen to them.
On liveaboards, you will have no trouble. The captains spend a lot of time with local tribal leaders in the areas they dive. They have to maintain good relationships with the locals and interactions there are mutually beneficial. Piracy in the form of bandits sneaking onto boats and stealing equipment does happen in some areas. You might see that in some areas your dive boat will post night watches. In Kavieng, some questionable boatsmen came to our liveaboard with some stolen dive tanks and tried to sell them to our captain. He refused to deal with what he knew was stolen stuff.
You will have a very safe dive experience there. On my many trips there, I have never had a problem on the boats or resorts. Have FUN!
