The pilot was inspecting areas of noxious weeds to check on the results of recent spraying. The helicopter struck a power line which severed the bubble windscreen. The wire then contacted the pilot's throat before being cut by the main rotor. The helicopter struck the ground tail rotor first about 35 metres beyond the point of collision. After extricating himself from the wreckage, the pilot swam across a river, walked 3 kilometres to a homestead and drove 15 kilometres for help. He was later admitted to intensive care in hospital. The pilot had been informed that there were power lines in the area, but was unaware of their precise location. He had conducted a survey flight prior to commencing the spraying operation, but had not sighted the particular cables. There was evidence to indicate that the helicopter had flown under the cables on at least one occasion while spraying. The cables, which spanned a river, were supported by poles which were 253 metres apart and which were probably outside the pilot's normal field of vision.