Two Qantas passenger aircraft were just seconds from colliding when they came within 220 metres of each other near Adelaide.
Collision avoidance systems were set off as a Sydney-to-Perth plane came too close to a Perth-to-Sydney plane over the Great Australian Bight on Friday afternoon.
The plane travelling from Sydney had been granted permission by air traffic control to climb.
An air traffic controller has reportedly been stood down over the incident.
Passenger Gary Martin witnessed the near miss.
“There was nothing reported; I just happened to look out the window somewhere in South Australia and saw a plane fly underneath,” he told reporters at Perth Airport.
The West Australian newspaper’s aviation editor Geoffrey Thomas said all passengers would have died if the planes collided.
“From an accident like that there would be no survival whatsoever,” he told the Seven Network.