A Jetstream 31 aircraft was inbound to Sydney runway 34, on descent from 6,000 ft to 4,000 ft, when severe turbulence was encountered. Whilst being subjected to sustained buffeting, the aircraft experienced uncommanded pitch and roll events. Both pilots responded on the controls, in an attempt to minimise the excursions. At the crew's request, the air traffic controller advised that their separation from a preceding Airbus A300, also tracking for runway 34, was six miles. Examination of recorded radar information confirmed that the required wake turbulence avoidance separation standard had been maintained. At the time at which the Jetstream 31 had experienced the upset, the A300 was 6.3 NM ahead. However, when closest to the point at which the upset occurred, the A300 had been 600 ft higher than the Jetstream, and the flight path of the A300 had been about 1 NM upwind of that of the Jetstream. The effect of the prevailing wind, and the descent of the Jetstream to an altitude of less than 1,000 ft below that of the A300, placed the significantly smaller aircraft in the wake turbulence generated by the wide bodied aircraft.