SYDNEY (AFP) - It was not quite a "Snakes on a Plane" scenario, but passengers on a Qantas jet watched in amazement as a three-metre python clung to the outside of their aircraft during a flight.

The Australian carrier said the flight from the Queensland city of Cairns to Port Moresby, capital of the Pacific island nation of Papua New Guinea, took off early Thursday morning with the unintended passenger tucked into its wing.

"The snake was seen by passengers once (the plane) reached cruising altitude," a Qantas spokesman told AFP.

"It was still on the aircraft when it arrived in Port Moresby but it had died by that stage."

Once they spotted it on the wing, passengers watched as the reptile engaged in a life-and-death struggle to maintain its grip on the plane despite the winds and chilly altitude temperatures for the two-hour journey.

Passenger Robert Weber told Fairfax Media on Friday that while people at the front of the plane were unaware of the python, those at the back were "all totally focused on the snake and how it might have got onto the aircraft".

Unlike the 2006 "Snakes on a Plane" movie starring Samuel L. Jackson, this reptile did not affect the flight.

"There was no panic. At no time did anyone stop to consider that there might be others on board," Mr Weber said.

He added that the snake had been nestled neatly at first, but once the wind caught the end of its tail, it was "pulling him straight out" and from then on it became a hopeless "life-and-death struggle".

"I felt quite sad for it, really," he said.

An expert said the snake was probably a scrub python, Australia's longest snake and one common in northern Queensland.

Qantas said it had never heard of anything similar happening before, adding that there was no way the reptile could have accessed the cabin.