The pilot had been mustering for 2.5 hours. He then landed to refuel the aircraft and collect the passengers. As the pilot subsequently attempted to climb the aircraft, out of ground effect, he noticed the rotor and engine rpm had decayed. The pilot attempted a run-on landing, however, the aircraft landed heavily, the main rotor struck the tail boom and the tail shaft was severed. The pilot did not report any engine or aircraft system defect which might have contributed to the accident. He had relatively limited experience on the aircraft type and in the type of operation being conducted. The pilot's description of a decay in rotor rpm accompanied by a decay in engine rpm indicates overpitching of the main rotor. The pilot's general experience coupled with the hot/heavy/high and relatively confined circumstances of this take-off would have contributed to his attempting to introduce more collective pitch than the power available was able to support. This accident was not the subject of an on-site investigation.