The aircraft were part of a four aircraft formation taking part in a photography session. On return to Darwin it was intended that the aircraft land one after the other on the same runway. The first aircraft landed and the pilot requested a clearance to turn off the runway onto a cross strip. A clearance was not available due to other traffic on that strip and the pilot was instructed to continue taxying along the runway. As the second aircraft commenced the flare for landing it collided with the aircraft already on the runway. The flight had been poorly planned and the briefing conducted before DEPARTURE was inadequate. The pilot flying the second aircraft to land was inexperienced on the aircraft type and had no formation flying experience.