The death of a helicopter pilot over Lake Wanaka in 2018 has been attributed to mast bumping.
Pilot Matthew Wallis died when the rotor blade of his Robinson R44 helicopter entered the cabin and quickly broke the machine up in the air.
Wallis, 39, was the sole occupant, and there were no witnesses and no in-flight recordings in the crash.
Turbulent conditions and speed were also likely contributors to the crash.
The final report by the Transport Accident Investigation Commission (TAIC) confirms the accident was probably caused by mast bumping, forcing the rotor blade into the cabin.
"The helicopter was likely to have encountered unexpected turbulence, and that speed may have been a factor," TAIC chief commissioner Jane Meares said.
"When passing over Stevenson's Island, [the helicopter] encountered an unexpected and catastrophic event, lost control, and fell into the lake."
Turbulence is a likely factor in five out of six Robinson helicopter accidents here over the past decade, the type of helicopter Wallis was flying.
"Mast bumping and the performance of that rotor head is the major concern," lead air investigator Ian McLelland said.
The Wallis aviation family endured a double tragedy in 2018. Just months after the helicopter crash, young Nick and two Department of Conservation workers were killed in a crash after a pair of trousers got tangled in a tail rotor.
"The Transport Accident Investigation Commission is urging New Zealand pilots of Robinson helicopters to be cautious when planning and conducting flights into areas of potential turbulence," Meares said.
TAIC still believes the machines are safe, provided they're maintained and flown in appropriate conditions.