A digital malfunction was to blame for a communication blackout at Christchurch air traffic control, an investigation has revealed.

In June 2015, they lost contact with aircraft under their control for four minutes, due to a programme update.

The back-up systems did not work as expected as the guidelines were out-dated.

A review out today by the Transport Accident Investigation Commission recommended the guidelines be updated.

The Ministry of Transport has agreed to review the guidelines to take into account technological developments.