The report found a worn latch likely caused a door to fly open, allowing a piece of loose clothing to be sucked into the tail rotor, which caused the crash.
Lou Sanson, who was the director-general of Doc at the time of the crash, said yesterday he remembered the 2018 crash with a "tremendous sense of loss".
The Alpine Group Ltd (TAGL) said it accepted the findings, but what led up to the fatal crash "will likely never be known".
One witness recalled seeing the helicopter descend near vertically, with items trailing behind it, moments before the crash, a Transport Accident Investigation Commission report released today said.
The crash killed Alpine Helicopters pilot Nick Wallis and Doc senior rangers Scott Theobald and Paul Hondelink, who were to be flown to a remote staging point for a wildlife culling operation.
Packed in the cabin were four rifles, 4000 rounds of ammunition, a chilly bin with food and drink, recording equipment for the wildlife cull, and two 20-litre plastic containers full of aviation fuel.
All three occupants had loaded their cold-weather overalls in the cabin.
When the left rear door opened unexpectedly shortly after takeoff, a pair of unsecured overalls flew into the tail rotor, making the helicopter uncontrollable.
It crashed and caught fire, killing the men on board.
Doors opening in flight were a problem, but at the time pilots had seen them as normal and not especially hazardous, Mr Davies Howard said.
"It’s a very sad, tragic accident ... we really do hope this report will encourage others to take door-openings seriously and secure all luggage inside."
The report said the entire accident took place over 14 seconds and the debris was dispersed over a 1km area.
It said the commission was advised after the accident of four previous incidents with the company of doors opening in flight.
The commission was concerned that the risk of doors opening in flight could become normalised to some pilots, the report said.
"This normalisation has led to door opening in flight being seen as not especially hazardous and therefore not worth reporting to the Civil Aviation Authority.
"If an unusual or hazardous event occurs in the air and it's not reported, maintenance engineers and operators won't be aware there's a problem to rectify."
The Taic report said the lessons learned from the "tragic accident" were that pilots needed to promptly report any unusual events when airborne and operators needed to enforce stringent cargo-securing practices.
It said aircraft manufacturers should provide clear instructions for maintenance and regulators should ensure regulations were clear to all concerned.
TAGL was sentenced in the Queenstown District Court in 2022 on two charges of failing to comply with its duties under the Health and Safety at Work Act.
It was fined $315,000.
Yesterday, the company put out a statement in response to the report.
"The time taken to conclude this investigation is now almost six years. However, it is important to note that the early findings and safety recommendations outlined in the commission’s interim report, released only a short time after the accident, undoubtedly had a positive effect on aviation safety.
"We are grateful for the considerable work conducted by Taic and acknowledge the commission’s analysis outlined in the final report.
"We welcome the report’s findings, accepting that the actual initiating sequence and cause of the accident will likely never be known."
TAGL said it did not own or maintain the helicopter, but had leased it for a short period as a fleet stopgap.
Mr Davies Howard said the report was about seeking to avoid similar accidents in the future.
The report said the incident should be taken as a warning for all pilots.
Mr Sanson said he was still filled with a "tremendous sense of loss".
"The three individuals’ contribution to conservation was immense. I admired all of them; they left a huge legacy.
"I will live with that accident for the rest of my life."
Doc director of health and safety Harry Maher said the department continued to monitor helicopter safety, incident management and reporting, and overall health and safety culture.
"When Paul, Scott and Nick were lost, Doc’s senior leadership vowed to do everything necessary to ensure all staff and contractors come home safe after work.
"We wanted to ensure the events leading up to the crash were investigated and changes made to ensure the safety of all staff and contractors."
