The pilot had been carrying out a series of joy flights at a rural field day. Refuelling was taking place from 200 litre drums, which had been placed in the shade of a large tree. On the second occasion that fuel was required, the pilot hover taxied to the drums, which were rolled out of the way on completion of the refuelling. As the pilot started to hover taxy again, the helicopter suddenly rose higher than anticipated and the main rotor struck the overhanging branches of the tree. One rotor blade de-laminated, severe vibration occurred, and the helicopter struck the ground heavily. With the helicopter hover taxying two feet above the ground, there was only three feet of clearance between the rotor blades and the tree branches. The reason the helicopter rose sharply and struck the branches was not determined, but may have been the result of a wind gust.