AIRASIA X pilots flew so low into Gold Coast airport on two flights that the safety margin from ground and other aircraft could no longer be assured, air investigators have found.

The pilots disregarded safe minimum altitudes for no apparent reason, investigators from the Australian Transport Safety Bureau said.

The low flying meant the airliners strayed into non-controlled airspace used by other planes.

That the errors occurred on different flights with different air crew little more than three weeks apart highlighted systemic gaps in the airline's pilot training, the bureau found.

The incidents occurred in rain, cloud and low visibility and involved multiple attempts to land.

The bureau found the pilots were probably not adequately equipped to handle non-standard landing approaches ''other than [in] autopilot managed mode''.

In the first incident, on May 4, 2010, on approach to Gold Coast airport for their second attempt at landing, the pilots in an Airbus A330 with 258 passengers on board, sank below the safe level above the ground.

At 20 kilometres from the airport the plane was already at the minimum allowable level but 900 feet lower than recommended. At the lowest, the pilots flew 200 feet below the official minimum of 1500 feet - and were alerted by air traffic controllers to their low flying, but did not regain altitude - before ultimately abandoning the landing at 750 feet above the ground.

The pilots abandoned a third landing attempt and diverted to Brisbane.

Investigators said it was ''not clear'' why the pilots flew too low, but unfamiliarity may have been a factor, investigators said. On the second flight carrying 260 passengers, on May 29, the pilots, about 19 kilometres from the same airport, began to fly under the recommended altitudes.

The similar breaches on both flights ''indicated there may be systemic factor[s]'' in the airline's training.

In response, AirAsia X revisited its pilot and instructor training, putting all pilots through Gold Coast-specific simulator sessions.