The pilot reported that both engines operated normally as he commenced the take off roll on runway 10. He said that at about three quarters of the way down the runway, as he raised the nose, the aircraft immediately veered to the left from what he recognised as a partial loss of power from the left engine. The landing gear was retracted but the aircraft climb performance was insufficient to prevent a collision with the top of a sand dune beyond the end of the runway. The aircraft continued over the dune and came to rest on the beach beyond. A subsequent visual inspection of the aircraft determined the left propeller blades were in fine pitch and the right propeller blades were in the feathered position. The reason for the reported loss of power from the left engine was not determined. However, only about 3 litres of oil remained in the right engine, with no obvious signs of oil leakage.