A final report into why an AirAsia X flight turned back after take-off from the Gold Coast last year has found a “rare” birdstrike caused its engine to burst into flames.

The Australian Transport Safety Bureau’s final report released on Wednesday, suggests a company pilot travelling as a passenger advised the flight crew of a "starboard engine fire" which was visible from the aircraft cabin.

According to the report, the engine met certification standards for the “ingestion of birds” and rather determined the incident as a rare scenario comprised of a “combination of aircraft and engine speeds, the height of the fan blade and its angle of incidence.”