The pilot reported that, soon after completing the pre-landing checks during the descent to land, the engine lost power. After checking that the mixture was full rich, he called Hamilton Tower to advise of his situation and was cleared for a straight-in approach. The pilot then changed the fuel selector from the left to the right tank for a short time, but did not actuate the electric fuel boost pump. When the engine did not respond, he re-selected the left tank and again attempted unsuccessfully to restart the engine. Moments later the aircraft made a forced landing into the sea approximately 2 km short of the runway. No fault was found with the aircraft which might have contributed to the accident. Only minute quantities of fuel were found in the fuel system and both tanks contained only sea water. The pilot stated that the aircraft departed Maroochydore with full tanks (337 L) with the left tank selected, and that he changed tanks every 30 min during the flight. He said he had not personally refuelled the aircraft but had noted the fuel level in both tanks prior to DEPARTURE to be at the bottom of the filler tubes. These tubes extend downward into each fuel tank from the filler caps a distance of approximately 10-12 cm. The pilot advised that much of the flight was conducted below 2000 ft with the mixture full rich or only slightly lean. Investigation determined that the actual fuel quantity on board at DEPARTURE was 270-280 L (135-140 L per tank). This meant that 55-65 L remained on board at the time of the accident. Judging from the duration of the flight and the tank selection procedure used by the pilot, the left tank should have been selected for 120 min (4x30 min periods) and the right tank for 90 min. Total useage from the left tank should therefore have been 110-120 L, still leaving 20-30 L in the tank when the power loss occurred. However, had the pilot missed a tank change and had the left tank been selected for 150 min instead of 120, the fuel quantity used from the tank would have been around 150 L, close to the calculated quantity in the tank of 135-140 L. It is likely, therefore, that the pilot missed a tank change, thereby draining the left tank, and that the engine lost power as a result of fuel starvation. Because the electric boost pump was not actuated when the pilot selected the right tank after the power loss, it would have taken more time for the fuel to flow from the right tank to the engine. The left tank was probably reselected before this had time to take place.