The snake, a scrub python, was spotted on the wing of the Qantas flight to Papua New Guinea around 6:15 a.m., The Sydney Morning Herald reported.
"The people at the front were oblivious to what was going on but the passengers at the back were all totally focused on the snake and how it might have got onto the aircraft," said passenger Robert Weber. "There was no panic. At no time did anyone stop to consider that there might be others on board."
Weber said people in the back of the plane watched as the flight became "a life-and-death struggle for the snake."
"I felt quite sad for it, really. For the remainder of the flight, he was trying to pull himself back into the plane, even though he was fighting against 400km/h winds. The cabin crew told us that at cruising altitude, it was minus 12 degrees outside -- but not even that was able to finish him," Weber said.
It was unclear whether the snake was found dead or alive when the plane landed in Port Moresby.
A Qantas spokeswoman said, "We have never heard of this happening before."