A road accident victim required urgent transport to a hospital offering specialist facilities for her particular condition. Road transport by a suitably equipped intensive-care ambulance was not available, and a decision was taken to employ a helicopter. When first contacted, the pilot expressed doubt about the feasibility of using the helicopter because of adverse weather conditions in the area. Some 30 minutes later the pilot was again contacted and requested to undertake the flight. He agreed, subject to weather conditions being suitable, and prepared for the flight from Wollongong to the hospital at Bowral where the patient was being held pending transfer to Sydney. The pilot, accompanied by a crewman and a paramedic, departed Wollongong at 0027 hours local time. As the aircraft approached Bowral, deteriorating weather conditions were encountered. Arrangements were then made through the ambulance radio network for the patient to be transported to Wilton, where the pilot landed shortly after 0100 hours. The patient arrived about 20 minutes later and DEPARTURE for Sydney was made at 0153 hours. When the pilot contacted Sydney Flight Service he was advised that the Control Zone was closed to visual operations because of low cloud. The paramedic considered that the flight should continue because of the patient's condition, and the pilot declared a mercy flight. He was cleared to continue on an emergency basis, with radar directions being provided to assist navigation. The reported cloud base at Sydney Airport was 300 feet, and the night was dark, with no moon. At 0212 hours the pilot reported that the aircraft was running into cloud, and he would hold over Parramatta. No further transmissions were received from the aircraft. Shortly afterwards it was discovered that the aircraft had crashed on the tidal embankment of the Parramatta River. A subsequent detailed examination of the wreckage revealed no mechanical defect or malfunction which might have made an accident inevitable. It was considered that the accident probably resulted from a loss of control of the helicopter during a turn away from an area of low cloud. Neither the aircraft nor the pilot was approved for flight in other than visual conditions, and the pilot probably became disoriented either from the loss of the visible horizon, or by reference to a false horizon. It was evident that he was in the process of regaining control, but insufficient height was available to complete the recovery before impact with the ground.