Soon after the helicopter had left Moomba on a charter flight to Whanto, the pilot reported hearing a loud bang and noticed that the rear left door of the aircraft, and associated pillar, were missing. The pilot reduced power, confirmed that his passengers were safe, and checked that the flight controls and systems were performing normally before turning to land at Innamincka Station homestead. The helicopter landed without further incident. After shutdown, the missing door was found impaled upon the horizontal stabiliser. The helicopter was configured to carry a stretcher by incorporating an approved kit, in accordance with Bell Service Instruction No. 206-68. With the kit correctly fitted, the co-pilot door could not be closed unless the pillar-locking handle was in the locked position. An additional safeguard of the system was that when the co-pilot door was closed and locked, the pillar-locking handle could not be moved to the unlocked position. Following the accident, the operator found that the stretcher pillar had unlatched in flight. An inspection revealed that the locking handle was in the unlocked position. However, the locking handle had been installed 180 degrees from the correct position, circumventing defences designed to pevent the helicopter departing with the pillar unlatched or being unlatched during flight. LOCAL SAFETY ACTION The operator has advised the Civil Aviation Safety Authority of possible shortcomings in the flight manual supplement, with regard to the correct operation of the litter kit. The operator has also incorporated a modification that includes a safety latch, preventing the pillar-locking handle from being incorrectly installed.