Pretty limited, mate, but I lived in Papua New Guinea for a couple of years, and tried to find out a little.
There are a massive number of different tribal affiliations. More languages by FAR on PNG, and it's not even close. Erm...upwards of 800, I think maybe Nigeria is next with 200-300? I would check facts, but the first place I went was here...
Languages of Papua New Guinea - Wikipedia, the free encyclopedia
And that has some seriously wrong info on it.
'Although it is an official language of Papua New Guinea, English is only spoken by 1–2% of the population'
Phhht...what a load of crap. It might only be 1-2% of the populations FIRST language...
Anyways, I digress. Christian missionaries (at least where I was living) had done a pretty good job of spreading the message. Most people where I lived were Seventh Day Adventists, Lutherans, Catholic or...hmm..trying to remember...Jehovah's Witnesses I think. But the first 3 groups was predominant. And I do mean predominant.
I was pretty careful about identifying myself as an atheist. I was in a remote area, education levels were exceedingly low, and everyone carried a machete...lol
However, as I grew more comfortable with some of the locals (I played basketball with them) I was a little more honest when they asked me about religion. They found it confusing as heck, rather than a problem. I think in the end they decided I didn't follow a religion, rather than didn't believe in God.
Getting them to open up about traditional beliefs was a little tricky though. Most of the white-men where I lived were either bosses (I lived in a timber camp, worked as a school teacher) or missionaries. But, there was certainly traditional practices occurring pretty much everywhere. My guess is they were discouraged by the missionaries, but they're certainly not banned or actively suppressed.
So...lessee...I saw the following;
Ancestor worship. I acquired some masks, etc, which I thought might represent various spirits or similar, but it turned out they represented ancestors.
Traditional witch doctor...actually have a couple of photos of him somewhere. Traditional medicine, and fairly dark magics are still playing roles in some areas. A guy got off for murdering a witch doctor while the guy slept since the witch doctor had made certain threats and acquired hair...or something... (fingernails?). It was allowed as self defence. NOT saying that's common...
Animism. Caught a fairly large snake which lived under my house (the area I was in had ZERO poisonous snakes, which was refreshing for an Aussie...lol) One of the other guys up there 'borrowed' it from me, and took it down to the workshop. I watched from the hilltop as a dozen local bolted out of the workshop, which I didn't quite understand. After all, I wasn't a local, and I knew the snake was harmless (released it back under the house later...I was the only house in that area NOT having rat issues...!!).
Turns out they believed the spirits of evil men would reincarnate...or maybe be trapped in (couldn't work that bit out) snakes.
I have no idea how wide spread these type of beliefs were in any detail. I was living in West New Britain, and I know Highland culture (for example) is quite different in some areas. But I think the basic tenets of ancestor worship, animism, and fairly dark traditional magic and medicine are fairly consistent throughout the country.
Some of the more remote parts on PNG (and even more so, Irian Jaya) are amazingly remote and isolated.