The pilot reported that while conducting an aerial inspection of cattle, he flew the helicopter through a descending right turn into wind. As he commenced to flare the helicopter at about 100 ft AGL, the engine RPM dropped rapidly. He lowered the collective control and wound on more throttle. There was no response from the engine. The helicopter rapidly developed a high sink rate which the pilot was not able to arrest completely before impact with the ground. The helicopter bounced twice and rolled onto its left side. Neither of the two occupants were injured. They escaped from the wreckage through the right cabin exit. The licenced aircraft maintenance engineer who recovered the helicopter from the accident site reported that he could not find a mechanical reason for the power loss. He said that the helicopter was being operated on mogas which is a more volatile fuel than avgas and in the higher temperatures, it tends to vaporize in the fuel line, causing an interruption of fuel to the engine. The helicopter was fitted with an approved emergency locator transmitter which operated, alerting the Search and Rescue Centre in Brisbane.