A helicopter pilot working on the Snowy 2.0 project heard a mechanical "screaming" noise shortly before crashing in the Kosciuszko National Park, safety investigators say.

The Australian Transport Safety Bureau released its preliminary report into the April crash which destroyed the Garlick UH-1H helicopter and injured pilot Philip O'Driscoll.

Mr O'Driscoll had also survived an earlier helicopter crash in Canada in 2005, which left him with serious burns injuries.

Pilot Philip O'Driscoll suffered serious injuries from an earlier helicopter crash in 2005. ( ABC News: Supplied )

In the month before the accident in the Snowy Mountains he used the helicopter to help fight the Tathra bushfire which destroyed about 65 homes.

The report described how prior to the incident Mr O'Driscoll was preparing to lift the motor of a drill rig as part of work on the proposed Snowy Hydro expansion when he was asked to delay his approach.

"As the helicopter started to climb, the pilot heard a loud mechanical 'screaming' noise," the ATSB report said.

"And he started making plans for an emergency landing.

"Almost immediately, the pilot also heard an audible alarm, followed by a noticeable yaw.

"Around this time, a light-coloured gas or mist was evident near the engine area of the helicopter."

Mr O'Driscoll transmitted a mayday call before coming down in the Yarrangobilly riverbed, south-west of the work site.

It took two hours for the pilot to be winched from the accident site. ( Supplied: Australian Transport Safety Bureau. )

Workers on the ground rushed to the wreckage with fire extinguishers and helped Mr O'Driscoll to get out of the helicopter before starting first aid.

The pilot of a second helicopter working on the same project dropped off extra workers to help while a third flew to Cabramurra to transport medical support staff to the site.

'Heroes' saved pilot after crash

It took two hours for the injured pilot to be winched from the accident site and be transported to a Canberra hospital, the report said.

"The immediate rescue efforts of the ground workers afforded the best opportunity to assist the pilot escaping the helicopter, conduct first aid and mitigate the risk of a serious fire," the report said.

The Garlick UH-1H helicopter was destroyed in the April crash. ( Supplied: Australian Transport Safety Bureau. )

Mr O'Driscoll's business, O'Driscoll Aviation, paid tribute to his rescuers in a statement released several days after the crash.

"To the awesome guys with Phil on the ground, thank you for ensuring his immediate safety," it said.

"You got him out of the helicopter and to a safe place where he could be winched out. In Phil's eyes you are heroes and he will be forever thankful for your actions."

Mr O'Driscoll is understood to be recovering well.

The ATSB is still examining the helicopter's fuselage and flight and engine instruments as well as its maintenance history.