During a Police search for an offender in scrub country, the helicopter was required to hover at about 100 feet over level ground in light wind conditions. After about 12 minutes in the hover with the co-pilot at the controls, it became necessary to re-position the aircraft slightly to the right and rear. To maintain adequate observation of the search area the pilot in command took over the controls and elected to back the aircraft into the new position. Whilst backing-up a slight descent developed. Power was increased but without effect. The pilot then applied full power and forward cyclic control in an effort to arrest the sink and fly away, but descent continued until the aircraft struck the ground heavily. The right maingear was broken and the fenestron shroud sustained damage. After the impact the aircraft bounced back into the air and the pilot maintained a low hover while the extent of the damage was assessed. He then flew the aircraft to the base camp and carried out a successful landing on fuel drums, which supported the right side of the aircraft. The initial slow descent during the back-up manoeuvre was probably the result of the pilot having just assumed control of the aircraft, coupled with a diversion of visual attention while moving towards the required position. The reason for the final, rapid descent could not be established. The aircraft manufacturer had no information relating to previous uncontrolled descents under the conditions which existed at the time, and therefore no procedures had been developed to counter the phenomenon.