For several days the pilot had been giving flying instruction to the aircraft owner operating from an airstrip located in a valley on the owner's property. The strip was aligned in an east - west direction with a downhill slope to the west. Wind conditions had been such that take-offs were being made towards the west with right hand circuits to the north so as to operate over the lowest surrounding terrain. The pilot was aware of a power line running from east to west parallel to the airstrip situated on a ridge about 700 metres to the north which the aircraft could remain well clear of while flying parallel to it on the downwind leg of the circuit. On the day of the accident the pilot had arranged to take another person for a familiarisation flight. The wind had changed to an easterly at about 10 knots so the take-off was conducted uphill. This required a longer climb out before the pilot could commence a left hand circuit to again operate north of the airstrip. He turned onto the downwind leg when about 120 -150 feet above ground level at approximately the same distance from the airstrip extended centre line as before. The circuit area was now displaced to the east where unbeknown to the pilot the power line changed direction by about 20 degrees to the south. After turning onto the downwind leg the pilot noticed the power line converging towards him from his right. Although evasive action was taken it was too late to prevent the right hand wing from contacting the power line resulting in loss of control and the aircraft dived into the ground. The pilot reported that the aircraft had been operating normally before colliding with the power line. This accident was not the subject of an on-scene investigation.