The Australian Transport Safety Bureau on Wednesday released a preliminary report into the December 2 tragedy in which a Goulburn-based instructor was also killed.

It revealed weather stations closest to the accident site near Marulan showed significantly different weather from one another, both one hour before and at the time of the accident, and the main rotor blade on the helicopter struck fuselage in flight before crashing to ground.

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The Goulburn weather station recorded winds from a north-westerly direction, up to nine knots, visibility greater than 10km, and a temperature of 25 degrees.

But at Moss Vale, north-east of the crash site, a special weather report was first issued at 3.38pm, due to a significant deterioration of conditions in the area which prevailed for several hours.

The training flight departed Goulburn shortly before 4pm with the chopper's last recorded automatic dependent surveillance broadcast detecting it descending into a valley in the Morton National Park at 4.11pm.

Wreckage examination indicated that the main rotor struck the helicopter's fuselage in flight, leading to a loss of control.

"As we move forward with the investigation we will consider a number of factors that could have contributed to a rotor blade impacting the fuselage in flight, such as turbulence, pilot input, engine issues and aircraft controllability," ATSB transport safety director Stuart Macleod said.

"To date the ATSB has attended the accident site twice for examination and evidence collection, examined the helicopter's engine and tail rotor driveshaft and interviewed relevant parties.

"As well as analysing weather conditions at the time of the accident, the ATSB will continue our detailed technical examination of the engine and other recovered components, including electronic devices."

The Robinson R44 helicopter was built in May last year and at the time of the accident had an estimated 150 hours total time in service.

During a periodic maintenance inspection of the chopper in November, and in line with a safety alert from the manufacturer, the engine's intake valves were inspected and reinstalled with new gaskets.

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