Passengers and crew members had to overpower a 17-year-old who got onto a plane with a shotgun and ammunition in Australia.

It happened as the aircraft prepared to take off from Avalon Airport, near Melbourne, ahead of a flight bound for Sydney on Thursday afternoon carrying about 160 people.

Police believe the teenager got onto the airport tarmac by breaching a security fence, before climbing the front steps to the plane.

Footage published by Australian outlet 7News showed the suspect being restrained by a passenger, while a member of ground crew and a pilot removed a utility belt containing tools that the suspect was carrying.  The pilot can also be seen kicking the shotgun away from the teen, who is wearing a fluorescent jacket.

"How is this possible?" someone onboard can be heard saying in the footage.

Victoria Police said the 17-year-old, who is from the nearby Ballarat area, was being held in custody.

Superintendent Michael Reid told reporters that passengers had noticed the teen was carrying a gun as he climbed the steps up to the plane.  "The male was overpowered by three of the passengers, at least," he said.

Supt Reid said the local force was in contact with counterterrorism police but that it was too early to establish a motive.  "No doubt this would have been a very terrifying incident for the passengers," he said, while commending the "bravery" of those who had overpowered the suspect.

Barry Clark, one of the passengers, told Australian public broadcaster ABC that the teen appeared to be dressed like an airport worker and was "agitated".  He 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."

No one was injured during the incident, police said.  Investigators located a car and two bags belonging to the suspect nearby.

Avalon Airport is exclusively served by Jetstar, a budget airline operated by Qantas.

In a statement to the BBC's US partner CBS News, the company said it was working with authorities to investigate the incident.  "We know this would have been a very distressing situation," a statement read. "We are sincerely grateful to the customers who assisted our crew to safely manage the situation."

Avalon Airport CEO^▸ (Chief Executive Officer) Ari Suss said the airport had reopened.