Two Qantas planes came within 700ft of a collision over the Great Australia Bight this morning.
Qantas Flight 581 operating from Sydney to Perth was climbing from 38,000ft to 40,000ft when its Traffic Collision Alert System alerted the pilots of an impending collision with Qantas Flight 576 operating from Perth to Sydney.
Both Airbus planes carry 300 passengers and crew.
Flight 576 was flying at 39,0000ft.
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 pilots of Flt 576 followed the alert system warning and descended to avoid a head-on collision.
Qantas has confirmed the incident and "loss of separation" over Adelaide.
In a tweet, Qantas said it could "confirm that two A330s have had a loss of separation over Adelaide. There was no impact to passengers."
The airline said that indications are that the loss of separation occurred when one of the Qantas aircraft received clearance to climb from Air Traffic Control.
“Our pilots followed standard operating procedures in re-establishing the required separation distance following the alert from the onboard notification system,” Qantas said in a statement.
Airservices Australia said it had reported the incident to the Australian Transport Safety Bureau and had notified the Civil Aviation Safety Authority.
It said updates would be provided to the ATSB and Airservices Australia would cooperate fully with any investigation.
The Australian Transport Safety Bureau has begun an investigation which will include examining radar and audio data and interviewing air traffic controllers and flight crew.
It said the incident occurred just after midday (AEST), 19 kilometres west of Adelaide, when the flight one plane had begun to climb from 38,000 feet to 40,000 feet after being cleared to do so.
“Soon after, the controller cancelled the clearance and the aircraft descended back to 38,000 feet,” the ATSB said.
An ATSB spokesman said it was gathering more information to determine if it was necessary to investigate the incident.