The helicopter was being operated in clear conditions, with a five knot South - Westerly breeze, and an outside air temperature of minus six degrees Celsius. After landing on a snow covered ridge, the passengers advised that they would require only a short period of time to complete their work. While waiting, the pilot maintained sufficient power to prevent the helicopter sinking deeper into the snow than approximately 45 centimetres. On DEPARTURE, the helicopter lifted normally clear of the snow into the hover, then turned approximately 45 degrees to the left, to follow the ridge line instead of flying direct across the valley to the next destination. During take-off, slight sink was encountered. This was controlled by an increase in collective pitch, but almost immediately further sink occurred. The pilot again increased collective pitch and forward cyclic control expecting to accelerate through the sink. The skids contacted the snow surface, with the right hand skid digging in deeper, causing the helicopter to roll to the right. In response, left cyclic control and full collective were applied , but the helicopter continued rolling to the right, allowing the main rotor blades to contact the snow, which caused the helicopter to overturn. The prevailing conditions would appear to preclude the possibility of power failure caused by snow ingested through the engine air intake. A full mechanical inspection was made of the helicopter and its engine, but no fault was detected. It is possible the aircraft was over-rotated, allowing the skid toes to contact the snow. The cause of this accident was not determined.