The pilot had been advised by the property owner to land on a strip about two kilometres from the homestead instead of the usual strip. The available strip length was 1400 metres which was adequate for the operation. The pilot reported that when the aircraft became low during the later stages of the approach, he applied power, but realised that the main wheels would probably pass through tall grass near the threshold. Just prior to touchdown, the pilot heard and felt a loud bang. Immediately after touchdown, the aircraft adopted a left wing low attitude before the propeller blades of the left engine and left wing tip contacted the ground. The aircraft slewed through 90 degrees to the left and ran off the strip. The investigation revealed that when the landing gear was allowed to pass through the long grass in the undershoot area, it struck a concealed mound of earth, seven metres prior to the threshold. The impact caused the left oleo leg to become detached from the aircraft.