The aircraft was conducting an instrument approach to runway 21 when a Boeing 767 was cleared for takeoff from runway 21. The Boeing had just become airborne as the landing aircraft crossed the runway threshold. At a height of about 20 feet the landing aircraft encountered severe turbulence. The most likely source of the turbulence was the jet efflux of the departing Boeing 767 (Thrust Stream Turbulence), despite the distance between both aircraft exceeding the separation standards for an aircraft landing behind a departing aircraft. The turbulence was reported as being so severe that both pilots in the landing aircraft were occupied in keeping the aircraft upright. Safety Action: Following this occurrence the Bureau reviewed similar local and overseas reports. As a result of the review the Bureau recommended to the Civil Aviation Authority that they: 1. review the Aeronautical Information Publication (AIP) with a view to defining a wake turbulence separation standard for medium category aircraft following medium category aircraft on approach. 2. review the AIP and the Manual of Air Traffic Services with a view to clearly defining separation standards with respect to "Thrust Stream Turbulence". 3. assess the need for special or re-classification of Boeing 757 aircraft for the purpose of wake turbulence separation stamdards.