The helicopter was being positioned at the North Rankin "A" Platform to transport passengers back to Karratha. When the helicopter arrived on the helideck it was landed into a strong south-westerly wind and positioned into wind with the pilots' eye-line between the two lines painted on the helideck. The pilots elected to shut down the engines when they were told that the passengers were delayed. They did not reposition the aircraft, so that it was offset from the wind, prior to shutting down the engines, as required by company procedures. Wind tunnel test results for the rig helideck platform show that turbulent airflow over the helideck can be expected under the wind conditions prevailing at the time of the accident. The pilots confirmed that the wind on the helideck was different to the free-stream air flow. They noticed that there was a strong updraft in the vicinity of the leading edge of the helideck. As a result of a safety audit conducted several months prior to the accident, the company pilots were required to land their helicopters with their eye-line at or very close to the outer of the two lines on the helideck. The helicopter was landed between the two lines with the main rotor disc protruding over the leading edge of the helideck. When the engines were restarted after the passengers were boarded, the blades were seen to rise during the initial couple of revolutions as they came into the view of the cockpit. At about the same time a noise was heard from the rear of the helicopter and a vibration was felt. The start was discontinued and inspection found that the tail rotor drive shaft had broken at the rear end. Subsequent investigation revealed that the main rotor had struck the drive shaft which had then failed. It is probable that the strong updraft in the vicinity of the leading edge of the helideck caused the advancing main rotor blade to rise and the retreating blade to consequently dip, resulting in the main rotor strike.