He was hoping to be reunited with the Yaifo tribe, ...
"On this occasion who knows if the Yaifo will do the same".
He noted that returning to his pickup location would not be easy, especially at his "advanced age".
"This would make them the remotest people in Papua New Guinea, and one of the last people on the entire planet who are out-of-contact with our interconnected world".
A helicopter dropped Allen off at Bisorio without a satellite phone, Global Positioning System or companion and he was due back in the country's capital Port Moresby on Sunday to travel to Hong Kong.
His agent Joanna Sarsby told the Daily Mail: "His wife Lenka has not heard from him, she is very anxious".
"Either I must paddle down river for a week or so - or enlist the help of the Yaifo, as I did last time; together we managed to achieve the only recorded crossing of the Central Range", he said.
"So, if this website or my Twitter account falls more than usually silent - I'm due back mid-Nov - it's because I am still out there somewhere". So, don't bother to call or text. He was travelling alone and wasn't carrying a satellite phone or any Global Positioning System devices.
Allen's older sister, Katie Pestille, told BBC it was "out of character" for Allen to miss his flight. I grow older but not wiser, it seems... "He told me last month, just before he set off, that he had no idea how they would receive him, or even if he would be able to find them in such a remote part of the country".
Mr Allen's friend, BBC security correspondent Frank Gardner, who travelled to Papua New Guinea with him twice previous year, said: "I would say the chances are that Benedict is going to be fine, I hope those aren't famous last words".
"He may not be a young man any more but he is very fit".
Considering that Allen claims to have made the "first outside contact" with the tribe, there is not much known about these indigenous people, who his agent described as "quite a scary bunch".
"Goodness knows what has happened".
On October 11, in a message of particular foreboding, he warned no one should come looking for him were he to disappear.
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".