The pilot decided to divert to Mudgee when deteriorating weather conditions were encountered near Narrabri. Upon arrival in the Mudgee circuit, he positioned the aircraft for a landing on runway 04. On the downwind leg, the before landing checks were completed by reference to a checklist. The aircraft was established on the final approach with all prelanding checks completed. At a height of about 200 feet, both engines lost power simultaneously. The aircraft touched down heavily in a field about 100 metres short of the threshold to runway 04 and was substantially damaged as a result of ground impact forces. The reason for the simultaneous double engine failure was not positively established. However, it is possible that when the pilot carried out the before landing checks, both auxiliary electric fuel pumps were inadvertently selected to the high setting rather than low. Consequently, when the throttles were moved towards the idle position on the final approach, both engines lost power due to an over rich mixture.