The pilot was involved in the management of the company. He had intended that the proposed flight be conducted in a Beech 58 by a company pilot. However, due to circumstances, he found he had to do the trip himself. Due to maintenance problems, VH-CFS was substituted for the Beech 58. This resulted in the flight departing for Menindee some two hours behind schedule. On the return flight, after some inflight calculations, the pilot realised that he had based his fuel calculations on the Beech 58 endurance and not on the Beech 55. He estimated that he would be able to reach Condobolin with only about twenty minutes fuel remaining. He advised that this was the only aerodrome within range with fuel available. Near top of descent, at 10,000 feet, the left main tank ran dry. The pilot then ran both engines from the right main tank. About six minutes later while passing 7000 feet on descent, the right main tank ran dry. The pilot feathered both propellers, with the intention of making an emergency landing into a nearby field. When he selected the landing gear down, the motor only ran for a short time, leaving him insufficient time to complete the extension manually. The aircraft struck three sheep just prior to touchdown, then slid to a halt in the field.