Phil O'Driscoll, 45, suffered serious injuries in the crash that destroyed the UH-1H chopper he was flying on April 17, with authorities saying at the time that he was lucky to survive.

The Australian Transport Safety Bureau's preliminary report into the incident says Mr O'Driscoll was working on the Snowy 2.0 scheme and preparing to lift the motor of a drill rig when a loadmaster asked him to delay his approach.

Mr O'Driscoll raised a lever to take the helicopter above the tree canopy while he waited, but as the chopper climbed, Mr O'Driscoll told investigators he heard a loud mechanical "screaming" noise.

He started making plans for an emergency landing and almost immediately heard an audible alarm before the aircraft rotated.

A light-coloured gas or mist was visible around the helicopter's engine area at the time, according to the preliminary report.

Mr O'Driscoll transmitted a mayday call and performed an emergency landing in the Yarrangobilly River, about 24km from the town of Talbingo.

Four workers on the ground rushed to the scene on foot as one of the loadmasters called for help.

When the workers arrived at the riverbed, they managed to get Mr O'Driscoll out of the aircraft and take him safely away from the wreckage to provide first aid.

They also found fuel leaking from the fuselage, which had detached from the tail boom of the aircraft, and used fire extinguishers to prevent a fire in the helicopter's engine bay.

The pilot of a second helicopter dropped off another three workers to help, while a third Snowy Hydro 2.0 chopper flew to Cabramurra to pick up a nurse and paramedic, who treated Mr O'Driscoll until a medical helicopter arrived to take him to Canberra Hospital.

"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 preliminary report says.

Mr O'Driscoll's company, O'Driscoll Aviation, released a statement praising the workers at the time of the crash, saying he considered them heroes.

The helicopter was destroyed, but the fuselage was not affected by fire.

The preliminary report says the safety bureau's investigation is ongoing and will include an examination of the fuselage and other equipment in the helicopter, its technical failure mechanisms, cabin safety and survivability factors, and the chopper's maintenance history.

The Snowy Mountains crash was not the first time Mr O'Driscoll has walked away from a helicopter accident.

An experienced pilot with more than 20 years' flying experience, Mr O'Driscoll was lauded as a hero after a near-fatal accident in Canada in 2005, when he crashed into a cliff to avoid hitting people.

The day before the April incident, he had been in the air fighting a fire that scorched close to 4000 hectares in Sydney.

He also helped tackle the blaze that ripped through the NSW seaside town of Tathra, destroying 65 homes, in March.