The pilot reported that he had descended with the intention of moving a mob of cattle when some ran under the helicopter. He turned to follow the cattle but then realised that he was travelling downwind. Power to maintain height was not adequate and the main rotor was overpitched. As a result, the helicopter struck the ground hard with a ground speed of about 20 kts. The pilot assessed the wind as south-easterly at 15 to 20 kt. Prior to recovery of the helicopter, the owner checked the engine and was satisfied that it was capable of normal operation. The fixed emergency locator transmitter was activated, but was not used or required for the rescue. The occupants walked to a nearby hill and used a citizen band radio to contact a truck which took them to a local mine.