Two Boeing 737s, one a Qantas plane and the other Virgin, were en route to Brisbane from Melbourne when they were ordered into a holding pattern about 91km from Brisbane Airport on the morning of July 29.
The planes were required to maintain a vertical distance of at least 1000 feet (305m) or lateral distance of at least nine kilometres.
But the planes breached those requirements.
The Australian Transport Safety Bureau (ATSB) says an investigation is underway to identify what was breached, who is responsible and how close the planes were flying.
“One of those (the vertical or lateral distance) were infringed,” an ATSB spokesman told AAP on Tuesday.
“We’re working to identify the exact details.”
However, a Qantas spokeswoman says the planes were flying one above the other. She said it was a Qantas pilot who observed the breach and alerted air traffic controllers.
Virgin Australia spokeswoman said flight crew in both aircraft were aware of the other’s location at all times. She said there was no safety risk to either plane.
“Flight crew in both aircraft were following air traffic control instructions regarding their respective holding holding patterns,” she said.
She said they would cooperate with the ATSB.