Passengers overcome armed teen trying to board plane in Australia

Passengers in Australia overpowered a 17-year-old boy armed with a shotgun and ammunition on Thursday as he tried to board a plane near Melbourne after sneaking into the airport.

The teenager entered through a hole in a security fence at Avalon airport, then made his way to a plane with about 160 passengers aboard, Victoria police superintendent Michael Reid told reporters.

"As he approached the plane, he's mounted or climbed the front stairs to the front of the plane," he said.

"At that point, passengers have identified that male was carrying a firearm. The male was overpowered by three of the passengers, at least".

No one was injured, and the police took the boy into custody.

The plane had been scheduled to fly to Sydney on a flight operated by Qantas budget offshoot Jetstar.

Police were in contact with counter-terrorism investigators but it was too early to establish a motive, Reid said.

"No doubt this would have been a very terrifying incident for the passengers," he said.

"Victoria Police really commend the bravery of those passengers who were over to able to overpower that male."

The boy appeared to be acting alone in the afternoon incident and was not known to police.

It is "very concerning that a person can firstly breach security, approach an aircraft and then make his way potentially close to being inside an aircraft with a firearm," Reid said.

Footage of the incident aired by Australia's Channel Nine showed a boy in a fluorescent green jacket -- similar in colour to those worn by ground staff -- being pinned to the floor by a passenger and a pilot.

A flight attendant walks past the melee carrying what appears to be the butt of a shotgun. 

Passenger Barry Clark, a wool shearer from a rural town in Victoria, said the teen was "dressed as a worker" or some kind of "technician".

He appeared to be "agitated", Clark told national broadcaster ABC. 

"Before we knew it, there was a gun -- a shotgun appeared -- and I was worried about there being shots," Clark said.

"All I could do was get the gun out of the way. And then put him in a hold and throw him to the ground until the police came," he added.

"I've been taught from a boy to be responsible and look after others and we're a sporting family, so I was quietly confident I could handle him."