Four helicopters had been arranged to transport twenty one Hamilton Island resort guests to Black Reef for a boat cruise. At Black Reef the helicopters were to be parked on a pontoon, which had recently been marked to accommodate three helicopters at the one time. The first helicopter to arrive (a Bell 222, VH-HIA) was parked in the centre position, then shut down with the rotor blades positioned along the fore and aft axis of the aircraft. The second to arrive was parked to the left of VH-HIA and shut down. The third helicopter landed and after off-loading passengers proceeded to a nearby smaller pontoon. The fourth helicopter (Bell 206, VH-HIL) arrived, parked, and as the pilot selected idle power he noticed that the forward rotor blade of the adjacent helicopter was moving towards the rotor arc of his own aircraft. The rotors became entangled and VH-HIL turned violently through 180 degrees. During the manoeuvre two passengers were ejected from VH-HIL and the main rotor, the mast, and a section of the roof, were torn from the aircraft. The pontoon had been marked about four weeks prior to the accident to allow the parking of three helicopters. Both VH-HIA and VH-HIL had been aligned by the pilots with the guidance marks. The investigation determined that with VH-HIA parked in the centre position, insufficient space was available to meet the published requirements to allow a Bell 206 type helicopter to be taxied to and parked on the pontoon. An inspection of VH-HIA found that the rotor brake was not in the 'park' position, nor were the rotor blades secured in any other way. The company had verbally briefed all Bell 222 pilots on the requirements to place the rotor brake in the 'park' position when the aircraft was parked on the pontoon. When the aircraft was parked at locations other than on the pontoon, the rotor brake was not normally placed in the 'park' position. On this occasion the pilot forgot to place the rotor brake in the 'park' position.