Hayden Bruce and Tom Sheather were conducting powerline inspections on behalf of Essential Energy near Canberra on April 13 last year.

The Oberon Aviation Services flight was flying low above trees when it entered a steep descent, dropping nose down, about three hours into the flight.

Witnesses, including an experienced pilot, said the plane had made a left turn before entering the descent.

Pilot Mr Bruce and powerline inspector Mr Sheather were killed.

The Australian Transport Safety Bureau released a final report into the incident on Friday following an investigation.

The bureau found a sharp turn to inspect a powerline had likely caused the Cessna to drop and enter a spin that could not be recovered from.

"This investigation reinforces to pilots the importance of managing airspeed and bank angle to minimise the risk of stalling," ATSB transport safety director Stuart Macleod said.

"This is particularly important when operating in close proximity to the ground, such as conducting low-level air work, as well as during take off and landing, as recovery may not be possible."

Oberon Aviation has amended its training and checking protocols following the incident.

The changes focus on threat and error management and situational awareness training.

The operator also intends to introduce a "manoeuvre margin" to take into account stall speeds associated with steep turns, and modify aircraft to include angle of attack indicators.

ATSB chief commissioner Angus Mitchell said the plane appeared to have hit a critical angle which caused it to stall.

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The plane had a slower speed compared to previous turns, which hadn't caused issues.

The plane was extensively damaged and both men died at the scene.

The investigation noted Mr Bruce had nearly 1000 hours of flight time recorded, 572 of which were in the Cessna.

The flight operator had noted that powerline survey work could be fatiguing for pilots and observers, as pilots must constantly adjust the plane and power level to allow observers to view the powerlines.

"The operation of the aircraft at low level and relatively low speeds also left little room for error," the report notes.

Oberon set work limitation for pilots with less than 200 hours of survey experience, limiting them to six hours of flight time per day.

There was insufficient evidence to conclude that fatigue had played a role in the crash.