Pilot Phil O'Driscoll had been about to airlift a drill rig motor being used for the proposed expansion of the Snowy Mountains hydro-electric scheme when he was forced to make the emergency landing on April 17 this year.

The 45-year-old, who has decades of flying experience, was winched to safety following the crash and flown to Canberra hospital with head and spinal injuries.

After hearing the 'scream', Mr O'Driscoll immediately made plans for an emergency landing, the Australian Transport and Safety Bureau said in a preliminary report released on Wednesday.

An alarm then sounded in the "Huey" helicopter and a light-coloured gas or mist came from the engine area, about the same time Mr O'Driscoll sent a 'mayday' call over the radio.

The 53-year-old helicopter then crashed through trees and came to a rest upside down in the Yarrangobilly River, about 24km from the town of Talbingo.

The tail boom of the helicopter broke away from the fuselage during the crash.

Mr O'Driscoll told investigators weather conditions were ideal in the valley, with a slight breeze and good visibility before the accident.

Investigators found Mr O'Driscoll had been told to wait by the on-ground loadmaster because the riggers needed more time to prepare the motor for lifting.

The pilot hovered about 700m from the work site and carried out a full systems check during the wait, which indicated the aircraft was operating normally.

Mr O'Driscoll was then told to recommence his approach to the site before the loadmaster again radioed to temporarily delay the lift, the report said.

The ATSB's investigation is continuing and will include an examination of the wreckage, a review of the helicopter's maintenance history, cabin safety and technical failure mechanisms.

Mr O'Driscoll narrowly escaped death in 2005 after his helicopter smashed into a cliff in the Canadian wilderness as he tried to avoid hitting people, News Corp Australia reported in 2010.

Australian Associated Press