WELLINGTON (AFP) - A deadly helicopter crash in New Zealand occurred when a loose pair of trousers flew out of the cabin and wrapped around the tail rotor, an investigation found on Thursday (Dec 20).

The bizarre circumstances around the crash in October that claimed three lives near the South Island resort town of Wanaka were revealed in a Transport Accident Investigation Commission report.

"There is evidence that a pair of over-trousers that had been packed in the cabin came out of the helicopter and became entangled in the tail rotor," chief commissioner Jane Meares said.

"Paint marks on the over-trousers matched the colour and profile of the tail rotor blades, and marks on a tail rotor blade match a zip and dome connector on the over-trousers."

Releasing its interim findings, the commission said the case underlined the danger of loose items in helicopter cabins.

The crash occurred on Oct 18 when the helicopter was taking off, claiming the lives of the pilot Nick Wallis and two conservation department workers.

In another twist, the pilot's brother Matt Wallis died piloting a helicopter in the same area just months earlier.

The commission also released an interim report into that crash on Thursday, finding the most likely cause was a main rotor drive-shaft failure that caused the helicopter to break-up mid-flight.