The helicopter, an Alouette II, was parked near a helipad at Newcastle Airfield. The Pilot, who was the sole occupant, started the engine and performed a lift-off. The helicopter hover-taxied at a low height along a grass taxiway before it commenced an uncommanded yaw to the left from which the Pilot was unable to recover. The helicopter impacted with terrain and sustained substantial damage. The Pilot, who was occupying the right-hand seat, was seriously injured in the accident. The accident caused a small grass fire which was quickly extinguished by individuals who came to the assistance of the Pilot.