The aircraft was engaged in crop spraying operations. The spray runs were being conducted in a north-south pattern across the area which was intersected by a three strand power line running east to west. The pilot advised that due to the cross wind conditions, it was necessary to fly close to the power line on each pass to adequately control the spray pattern. On the sixth spray run, the main wheels struck the uppermost strand of wire, causing the aircraft to descend into the crop. The pilot was not injured in the impact, and was able to exit the aircraft safely. The pilot stated that on the last spray run he either misjudged the height of the wires or the aircraft encounted a down draft in the lee of tall timber growing along the paddock boundary.