Authorities are investigating a near-collision between two Qantas aircraft at Sydney Airport last year.

The Australian Transport and Safety Bureau on Thursday issued a preliminary report into the incident on the evening of August 5 during which the two planes came "very close" to a mid-air crash.

The report says that an Airbus A330, which was taking off en route to Melbourne, and a Boeing 737, which was coming in from Brisbane, came within 152m vertically and 796m laterally of each other.

'Serious incident'

It has been deemed a "serious incident" by the ATSB after the Airbus A330's collision avoidance system issued an alert.

"The A330 first officer, who was pilot flying, then saw the 737 in close proximity and, in response, reduced the aircraft's angle of bank to reduce the turn towards the 737," the preliminary report said.

"The captain of the A330 radioed to advise the ADC-E controller that their proximity to the 737 was 'very close'. The controller then issued an instruction to the A330 flight crew to turn left."

Trainee controller

The near-miss occurred after the Airbus A330 was cleared for take-off and as the Boeing 737 came into land.

An "otherwise experienced" trainee controller, who was under the supervision of an on-the-job training instructor, determined there was insufficient runway space between the two aircraft and instructed the Boeing 737 to "go around".

In the video below: Qantas plane bound for Sydney from Perth has been forced to turn back shortly after take-off

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The landing craft was told to turn right and during the missed approach the craft came too close.

The Airbus A330 went onto Melbourne without incident and the Boeing 737 landed a short time later after a second approach.

The investigation will include an examination of controller training, air traffic control procedures and instrument departure systems and the final report is due to be handed down mid-to-late 2020.

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"Preliminary reports outline basic factual information established in the early phase of an investigation," ATSB director of transport safety Dr Stuart Godley said.

"They do not contain findings, identify contributing factors or outline safety issues and actions, which will be detailed in an investigation's final report."