A pilot was heavily intoxicated when he took a helicopter and flew across a popular tourist spot before fatally crashing into a hotel roof.
However, Blake Wilson's decision to take the unauthorised flight in far north Queensland remains a mystery to safety investigators.
Mr Wilson, 23, had been drinking with friends at his farewell party in Cairns in August after taking on another role with his employer Nautilus Aviation.
He headed home to his apartment about 11pm before driving to the company's helicopter hangar at Cairns airport about two hours later.
The investigation found Mr Wilson used a security pass to access the hangar and CCTV footage showed a Robinson R44 helicopter taking off after 1.30am, flying for less than 20 minutes before crashing.
Australian Transport Bureau Safety Chief Commissioner Angus Mitchell said Mr Wilson was "heavily intoxicated" at the time.
Mr Wilson flew the helicopter over the CBD, his apartment building twice and along the marina before heading back towards the city centre.
The flight's last moments, caught on CCTV, showed the helicopter pitch up before immediately descending steeply into the roof of DoubleTree by Hilton in the Cairns CBD about 2am.
Parts of the rotor blade were found in hotel rooms, with other debris strewn across the venue's grounds and on the street.
The crash prompted the evacuation of 400 guests with two elderly visitors treated for smoke inhalation.
Mr Mitchell said it was fortunate no hotel guests were seriously injured.
The safety bureau found there was no mechanical failure that contributed to the flight.
"The flight was a purposeful act but there was no evidence available to explain the pilot's intentions," Mr Mitchell said.
He said Mr Wilson took "significant measures" to conceal the nature of the unauthorised flight.
Mr Wilson had a valid New Zealand helicopter licence with 157 hours of flying experience, about 16 hours of those in the R44 chopper.
There was no indication in Mr Wilson's logbook that he had flown a helicopter in Australia.
Mr Wilson did not hold the appropriate qualifications to fly the helicopter at night, nor did he have any experience.
He had flown the helicopter at 500 feet over the CBD, instead of the regulated 1000 feet.
Nautilus Aviation had earlier said Mr Wilson had been employed for four months as a ground crew member and was not authorised to fly the helicopter.
Mr Wilson had been due to start a new role with the charter company at Horn Island driving a fuel truck.
Mr Mitchell said the crash was an "exceptional set of circumstances" and would not normally warrant a safety bureau investigation given it was not an aircraft failure.
No safety recommendations were made.
"While it was tragic that the young man lost his life, there was significant risk to others here in the Cairns area, which is why we have put resources to find out what we can, look at all the available evidence," Mr Mitchell said.
"It is fair to say that this is quite an exceptional set of circumstances for the ATSB to be investigating."
