A Qantas aircraft flew too low on approach to Canberra Airport at night when the pilots momentarily lost track of their height.

A report on Friday from the Australian Transport Safety Bureau says the pilots lost ''situation awareness''. This is blamed for crashes when a pilot does not have a mental picture of the aircraft in relation to the ground.

The Boeing 737 was flying from Sydney to the ACT in February last year.

''The flight was at night with rain showers and scattered cloud in the Canberra area,'' the report says.

However, the instrument landing system which projects an electronic beam to guide aircraft to the runway threshold was not working due to scheduled maintenance. As a result, the pilots were following a step-down procedure to keep the aircraft safely above surrounding hills.

The 37-second deviation from the specified landing profile is blamed partly on a high workload and possible tiredness.

A Qantas spokesman said: ''The ATSB's report finds that the pilots took swift corrective action after the aircraft descended below the required altitude for that stage of the approach, before landing normally. There was no safety risk at any point.''

The flight crew were on day three of a four-day pattern and said they were adequately rested before beginning duty that day in Adelaide.

Their duty was described as long and particularly busy due to widespread thunderstorm activity.

''The captain reported that he and the first officer were tired after the busy day,'' the report says.

''The crew prepared for an alternate instrument approach that provided for lateral but not vertical flight path information.

''Shortly after becoming established on the final approach course with the aircraft's automatic flight system engaged, the flight crew descended below the minimum safe altitude for that stage of the approach.

''The crew identified the deviation and levelled the aircraft until the correct descent profile was intercepted, then continued the approach and landed.''

A minimum safe altitude is specified around an airport like Canberra to keep aircraft above surrounding hills during landing approaches.

''Although the consequences were not serious, any deviation from instrument approach procedures increases operational risk,'' the report says.

''In this case, the crew identified the noncompliance and took immediate action to resolve it.

''The ATSB found that at the time of the occurrence the automatic flight system was in the level change mode rather than the vertical navigation mode specified by the operator for such approaches.

''While in that mode the flight crew had selected an altitude lower than the applicable minimum safe altitude, with the effect that unless the crew intervened, the aircraft would descend to that lower altitude.

''The flight crew then allowed the aircraft to continue descending in the level change automatic flight mode through the segment minimum safe altitude, reflecting a temporary loss of situation awareness.''