During a cattle muster along a laneway, the pilot positioned the helicopter to head off a beast which broke away from the mob. As he descended in front of and to one side of the beast, he felt a sudden vibration through the helicopter and at the same time noticed an overhead powerline. He lowered the collective for an immediate landing. The landing which followed was heavy, spreading and breaking the skid gear. The helicopter then rolled onto its side. Post flight inspection by the pilot revealed that the tail rotor had struck the wire. The powerline was a single wire earth return line strung about 35 ft above ground. The span was reported to be 800 m long. The wire was struck some 100 m from the nearest pole which was partially hidden among foliage.