Police were called to Avalon Airport near Geelong, in Victoria, on Thursday afternoon following reports a male with a firearm had attempted to board the Jetstar flight.
A 17-year-old acting alone had climbed through a hole in the airport's security fencing before making his way on foot to a Jetstar flight, Victoria Police Superintendent Michael Reid said.
The flight which was carrying about 150 people was bound for Sydney.
The teen tried to climb the front stairs into the plane cabin but was noticed by passengers carrying the shotgun and subsequently overpowered by three civilians.
"This would have been a very terrifying incident for the passengers of that plane, and Victoria Police really commend the bravery of those passengers who were able to overpower that male," Supt Reid told reporters on Thursday.
Police detained the teenager and found live ammunition rounds along with the shotgun, but say he is not known to police.
Supt Reid said detectives were still investigating the motive for the incident and are speaking with counter-terrorism officers but said it is too early to deem the act terror-related.
They believe the boy climbed through a hole in the fence but whether he'd cut it himself or it was already there is being investigated.
The vehicle that he arrived in has been located, Supt Reid said.
Jetstar is working with Victoria Police and the airport to understand what happened.
"The safety of our passengers and crew is our number one priority and we can confirm there are no reported injuries," an airline spokeswoman said.
Avalon Airport is Victoria's second busiest airport and is closed to visitors until further notice, Avalon Airport chief executive Ari Suss said.
All flights scheduled for Thursday evening have been cancelled and re-routed.
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