A 17-year-old male has been arrested after breaching a security fence at Avalon Airport and entering a Jetstar plane with a loaded shotgun.
Police say they responded to reports a man attempted to board a commercial flight at the state's second-biggest airport about 2:20pm on Thursday.
The young man from Victoria, who was dressed in high-vis similar to airport ground crew, was tackled to the ground by passengers on flight JQ610 bound for Sydney and held down until police arrived.
Officers took him into custody to be interviewed, and have said he would likely be charged with federal and state offences.
Superintendent Michael Reid told reporters on Thursday evening the teenager, acting alone, had entered Avalon Airport via a hole in a security fence.
He said the teen made his way to the plane and walked up the stairs but once he was just inside the plane he was overpowered by several passengers.
"No doubt this would have been a very terrifying incident for the passengers of that plane and Victoria Police really commend the bravery of the passengers who were able to overpower that male," Superintendent Reid said.
He confirmed the youth was carrying a loaded shotgun.
He said police were talking to Counter Terrorism Command but the incident was not yet being treated as terrorism.
"It's too early to establish that," he said.
Asked about the significance of the security breach, Superintendent Reid said it was "very concerning that a person can approach an aircraft and make his way, potentially close to being inside an aircraft, with a firearm".
He said the teenager was not previously known to police.
Police later found two bags and a vehicle belonging to the boy.
Passenger hails 'Barry Clark in 1C' for taking action
A passenger on the flight named Woodrow credited his fellow passenger sitting at the front of the plane, "Barry Clark in 1C", for tackling the young man to the ground.
"We were all boarded, I was last on, sat in my chair, heard a kerfuffle and saw the pilot and this other guy tackling this young boy in a high-vis vest which, from a distance, looked like he was ground crew," he told ABC Radio Melbourne.
"He [was] dressed up with a tool belt and cargo pants and a high-vis yellow vest."
"In a split second, the gentleman Barry Clarke in 1C saw him and jumped him, tackled him and got him in a choke lock, disarmed him, pilot jumped in too, myself and another gentleman come up to support and hold him down."
Barry Clark, a shearer from Echuca, told ABC Radio Melbourne he first noticed the young man when he realised a female flight attendant was questioning him.
"He was dressed up as a worker and he got agitated and before we knew, it there was a gun — shotgun appeared — and I was worried about there being shots so all I could do was get the gun out of the way, threw it down the stairs — try to — and then put him in a hold and throw him to the ground til the police come," he said.
"He had other apparatus on him so I didn't know what his full intention was."
Asked what was going through his mind when he tackled the young man, Mr Clark said: "You don't think, you act."
"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 but I didn't want to see that poor girl get shot so you've just got to do what you've got to do," he said.
Mr Clark said the young man with the gun was dressed up to avoid attention.
"He looked like a technician and the lady that stopped him was the lady checking all the tickets and he couldn't come up with the credentials so then he got agitated."
Mr Clark said he spoke to the boy for about 10 minutes until police arrived.
Jetstar issued a statement shortly after the incident saying it was aware of a security incident at Avalon Aiport and was working with police to "urgently understand what has occurred".
"The safety of our passengers and crew is our number one priority and we can confirm there are no reported injuries," a spokesperson said.
Avalon Airport said it was investigating the situation and its immediate focus was the safety of passengers and staff.
A spokesperson for the airport said the matter was being handled by police and Avalon Airport would be closed until further notice.
"Passengers involved are now allowed to leave the airport. Avalon are assisting people with transport to Beach Road if they aren't parked at Avalon or aren't using SkyBus."
