A pilot flying a helicopter from Casino to the NSW Central Coast ascended to more than 3000 feet before witnesses described a rapid descent into a riverbank where he died.
The pilot, the sole occupant of a single-engine, five-seat chopper, crashed about two-and-a-half hours into the flight.
He died and the helicopter was destroyed after it plunged into a Hunter Valley riverbank, near the town of Maitland.
Australian Transport Safety Bureau (ATSB) investigators attended the scene and conducted an initial assessment of the aircraft before it was moved to higher ground for further examination due to rising water levels.
The preliminary report notes recorded data indicated the pilot was following inland visual flight rules but when approaching Tocal, the aircraft started to climb, and conducted a right 180-degree turn.
"Data showed that over the next 20 minutes, the helicopter conducted a number of turns, a climb up to 3100 ft, and a descent to around 120 ft above ground level," the report said.
"It was then observed by six witnesses, who reported it heading towards the Hunter River, descending slightly, and possibly initiating a turn when it rolled markedly and descended rapidly, colliding with the riverbank."
ATSB investigators said there were no pre-existing defects in the aircraft's engine or body.
The preliminary report has no analysis or findings, which will be detailed in the final report.
