Staff found the 20-centimeter (8-inch) Mandarin Rat Snake in the passenger cabin near the door late Sunday before passengers were due to board the flight bound for Tokyo from Sydney International Airport, Qantas said in a statement.
Australia's flagship airline said passengers were given hotel rooms overnight and left Sydney on a replacement plane Monday morning. Qantas said the original jet would be fumigated before returning to service in case there were other snakes on board.
The snake was taken by quarantine officials for analysis.
The Agriculture Department said the snake, a species that grows to an average 1.2 meters (4 feet), had been euthanized, "as exotic reptiles of this kind can harbor pests and diseases not present in Australia."
The department said the snake had arrived aboard the jet in a flight a day earlier from Singapore.
"The Department of Agriculture is looking into how the snake came to be on the plane, but isn't able to speculate at this time," it said in a statement.
The mildly venomous Asian snake was about the width of a pencil and did not pose a threat to humans, but it had the potential to cause ecological havoc in the Australian environment if it had escaped the plane with a mate, Canberra Reptile Zoo herpetologist Peter Child said.
While snakes rarely pose aviation hazards, a 3-meter (10-foot) python in January clung to the wing of a Qantas flight from the northeast coast city of Cairns to Papua New Guinea. The python died during the flight but was still attached to the wing when the two-hour flight ended in the national capital Port Moresby.
The 3-meter (10-foot) python fought to stay on the wing, pulling itself forward only to be pushed back by the frigid wind.
Passenger Robert Weber videotaped the struggle and told Australia's Fairfax Media that the wind whipping the snake against the side of the plane left a bloody smear.
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