Benedict Allen was dropped by helicopter into the remote jungle three weeks ago and has not been heard of since.
He was hoping to reach the Yaifo, a tribe thought to be one of the last on Earth to have no contact with the outside world.
The author and TV presenter, who has made six TV series for the BBC, has no mobile phone or GPS device with him and was expected to begin his journey home at the weekend.
Mr Allen’s sister, Katie Pestille,said: “He knows all about that survival stuff. It’s just what worries me is there are bad people in these jungles.
“You would think that they were totally empty but there are people in there. I mean, I know more about the Amazon, but there are loggers and drug dealers and all sorts of bad people.”
Mr Allen’s friend, BBC security correspondent Frank Gardner, who travelled to Papua New Guinea with him twice last year, said: “I would say the chances are that Benedict is going to be fine, I hope those aren’t famous last words. Benedict always expected something like this. I had supper with him just before he left and he said, ‘look, I’m quite certain I’ll probably be out of contact for quite some time and people shouldn’t worry about it’.”
Mr Allen’s agent Joanna Sarsby said his wife Lenka was “very worried”.
She added: “He is a highly experienced explorer, very clever and resourceful and adept at surviving in the most hostile places on Earth, and he would never give up. He may not be a young man any more but he is very fit.
“He was trying to reach the Yaifo people, a very remote and reclusive tribe – possibly headhunters, quite a scary bunch. Goodness knows what has happened.
“I just imagine he might have been taken ill or is lying injured somewhere, perhaps with a broken leg, and maybe being helped by locals.
“He never takes a phone with him – he believes in living like the locals. For him not to come back is really odd.”
In a blog post on his website, Mr Allen wrote in September: “The Yaifo are one of the last people on the entire planet who are out of contact with our interconnected world.
“In October I’m hiring a helicopter to drop me off at the abandoned mission station, Bisorio – a forlorn place.”
A Foreign and Commonwealth Office spokesman said: “Our staff are assisting the family of a British man who has been reported missing in Papua New Guinea, and are contacting the local authorities.”