Before commencing spraying, the pilot conferred with the farmer who pointed out three sets of power lines. He checked the location of each of these lines in relation to an adjacent highway and a tree line in the area to be sprayed. He also completed an aerial survey. The pilot successfully sprayed the first three quarters of the paddock and then commenced on what he considered to be the more difficult section. As the aircraft descended over the line of trees to spray height, the pilot heard and felt a thump. He flew the aircraft back to the strip, where he discovered damage to the leading edge of both wings. The aircraft had struck a fourth set of wires. This two strand set was connected to a cross tree on an 11,000 volt pole, and strung for about one kilometre through trees to a pole beyond the highway. The farmer had neglected to mention this pair of wires and the pilot did not see it on his aerial survey.