The PNG Orchids have leapfrogged Canada and France into fourth place in the women's standings following their 20-16 win over England on Saturday.
Shirley Joe scored a last-minute try to break the 16-all deadlock and secure PNG's first-ever test match win.
The Kumuls then overcame a half-time deficit to take down Great Britain, with the 28-10 triumph sealing their first win against the Lions in 29 years.
Head coach Michael Marum said it was one of the best games they have ever played.
"After last weekend's loss by two-points to Fiji, they were disappointed about that game but we came back home on Sunday to prepare for this big game and to see the way the boys finished off in that second half, it's probably one of our best games ever," he said.
With a four-point deficit at the break, Marum said their last 40 was built on effort.
"At half-time, [we discussed] how will we represent and it was our last international match of the year and a lot of us wont see each other for six months until you have your first international match so we asked for efforts there so you know the boys did well in that."
Great Britain finished winless in their first international tour for 12 years. Although his side has been heavily criticised for under-performing, Lions coach Wayne Bennett said that international rugby league was on the rise.
"With the international game it's a credit of the international game as we go into five or six years now and starting to see the benefits of it when they're playing more test matches and more competition," he said.
"You've seen the emergence of Tonga, you see how much your team [Kumuls] have improved, Samoa's much improved, the boys all want to play for their country now. So, whilst we've got to swallow a bit of pill, on the other side of it, I've just seen the game getting stronger."
The Kumuls win over the Lions lifted them to sixth in the men's world rankings, leapfrogging Samoa, France, Scotland and Lebanon.
New Zealand has knocked world champions Australia off the top of men's standings following their 2-0 series triumph over Great Britain, while Tonga remains in fourth despite its historic wins against Great Britain and the Kangaroos.
Australia, New Zealand and England remain unchanged atop the women's rankings.