Two Qantas passenger planes were forced to take action after getting too close to each other west of Adelaide.
Qantas says the two Airbus A330 aircraft had a "loss of separation" on Friday.
One was travelling from Sydney to Perth when it was granted permission by air traffic control to climb.
The other flight was travelling from Perth to Sydney.
"Our pilots followed standard operating procedures in re-establishing the required separation distance," the airline said in a statement.
Qantas said there was no impact to passengers on either plane.
The Australian Transport Safety Bureau (ATSB) 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 crew of one plane was cleared to climb from 38,000 feet to 40,000 feet and had begun to climb.
"Soon after the controller cancelled the clearance and the aircraft descended back to 38,000 feet," the ATSB said.
It said the flight crew of the second plane received a resolution advisory alert from the aircraft's traffic collision avoidance system, requiring immediate action.
The ATSB said it expected to complete the investigation of the incident this month