Two Qantas passenger planes got dangerously close over southern Australia and triggered a collision warning alert.
Australian authorities are investigating the incident which forced the pilots to take evasive action.
The near miss happened on Friday and involved two Airbus A330s travelling in opposite directions between Sydney and Perth, AFP reports.
An eastbound plane was reportedly cruising at 39,000 feet when a westbound plane was given permission to climb from 38,000 feet to 40,000 feet, triggering one of the plane's traffic collision avoidance systems, the Australian Transport Safety Bureau (ATSB) said.
Richard Woodward, vice-president of the Australian and International Pilots Association, told ABC that typically the alert would "give you a warning about 25 seconds out, so theoretically they were anything around 25 or 30 seconds apart when the aircraft got the warning.
"At that altitude and that speed, it's very difficult for the crews to see and take avoiding action.
"If the collision warning computer goes off, it is effectively the last line of defence. It means the system's broken down.
"If they were cruising in cloud, there's no chance of seeing each other. If you're in clear air, there is a reasonable chance of seeing each other.
He added: "They missed by a reasonable distance. It doesn't sound like a lot, about 700 feet or so."
But the ATSB says it is too early to confirm the distance between the planes.
"An investigation team is put together of suitably qualified and experienced people and they will be talking to the pilots involved, the air traffic crew, gaining recorded information from the aircraft," the spokesman said.
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