A helicopter that crashed killing two people in Mt Aspiring National Park plunged 30 metres down a mountain after its rotor blades struck the rockface, the Transport Accident Investigation Commission has found.

Pilot Tim Brown and guide Sean Roach died in the crash on May 17 this year.

The Airbus AS350-B3 helicopter belonged to The Alpine Group, which is owned by the Wallis family.

The company said at the time both were "highly experienced and valued members of a close-knit team".

It was one of two choppers being chartered for a hunting trip that day, with the second helicopter carrying a guide and four passengers.

In interim findings released by TAIC on Thursday, the lead helicopter was hovering by a rocky ledge as the pilot and hunting guide tried to recover a shot mountain goat or chamois.

"The pilot and the guide flew up the mountain face to retrieve the chamois," the report said.

"The [cockpit camera] recording showed the accident helicopter was hovering by a rocky ledge. The guide was exiting the accident helicopter through the front left door when the accident sequence started. Marks on the rocks and tussock grass at the accident site showed that the main rotor blades had contacted the mountain face."

The report said the cockpit camera and a photograph taken by one of the party suggested the accident happened at 9.35am.

The weather was frosty that morning, but the forecast was for a fine day with little to no wind and clear skies, the report said.

Earlier, the two helicopters flew to an area 24km west of Makarora, where the hunting party spotted a chamois on the north face of Mercury Peak.

The guide and hunter were off-loaded from the lead helicopter nearby.

"The pilot then flew the accident helicopter a safe distance away. The second helicopter also remained clear at this time. The hunter shot the chamois, which fell into a gut," the report said.

"The pilot of the accident helicopter returned and uplifted the guide and hunter, flying them a short distance to a large flat area. The hunter and two passengers were offloaded to await the arrival of the second helicopter."

One of the offloaded passengers from the main helicopter was observing the recovery of the chamois and took a photograph of the accident helicopter moments before the accident.

They then saw the helicopter falling down the mountain face and coming to rest below.

The hunter and two passengers alerted the pilot and guide of the second helicopter after it landed in an area just out of view of the accident a few seconds later.

"The second helicopter pilot immediately informed the operator of the accident, who activated their emergency response plan and contacted emergency services."

Later that day, members of the alpine cliff rescue team and police recovered the bodies of Brown and Roach.

A full inquiry into the accident was continuing with a final report, setting out findings, safety issues and recommendations, to be issued when completed.

The Alpine Group is run by brothers Toby and Jonathan Wallis, sons of the late aviation pioneer Sir Tim Wallis.

Two of Sir Tim's sons, Matthew and Nick, died in separate helicopter crashes in 2018.