Pilots of the Qantas Airways flight stopped the plane. The man was coaxed out of the wheel well and arrested for trespassing, said airport spokeswoman Nancy Castles.
"This was an extremely dangerous thing for him to do. If he had continued to cling in there with the aircraft taking off at over 200 miles (320 kph) per hour, he might have fallen out and could have been sucked up by an engine," she said.
"If he had survived that and was in the wheel well when the landing gear was retracted, he could have been crushed by the mechanism. And if not he very likely would have frozen to death during the 15 1/2 hour flight at 30,000 feet (9,150 metres) while wearing no clothes."
The man, Neil Melly, 31, tried to buy a one-way ticket on the Qantas flight on Monday evening, but was turned down because he could not supply a valid credit card, Castles said.
Later, he managed to climb over an airport fence, topped by three strands of barbed wire, without injury and was spotted by a ramp worker "running, naked, full-speed" toward the plane.
Castles said a check by authorities found that Melly had been reported missing to the Royal Canadian Mounted Police and was suffering from bi-polar disorder, a manic-depressive illness.