A helicopter crash at Auckland's Viaduct Basin last month was caused when a supervisor pulled on a cable which hit the aircraft's rotor blades, a report has found.

The preliminary Civil Aviation Authority report said "massive out-of-balance forces'' tore through the helicopter after it struck a lifting line on November 23.

The helicopter was being used to raise a 25m tower for the Telecom Christmas tree.

After lifting the tower, the aircraft lowered and hovered at about 5m so the line could be removed from its hook by the rigging supervisor on the ground, said the report.

"When the rigging supervisor jumped up to grab the line, it instantly tightened and touched the helicopter's main rotor blades. The force of the impact caused major structural damage and the aircraft hit the ground,'' it said.

Pilot Greg Gribble walked away from the spectacular crash, and there were no other injuries.

An ongoing investigation will look at the pre-flight safety briefings, pilot's experience and training, health and safety aspects, possible preventative measures, and any human factors that applied.

A full incident report could take up to 12 months.

- APNZ