A flight that overshot its destination by almost 50 kilometres because the pilot fell asleep would have crashed if the operator did not wake up in time, an aviation expert says.

Key points: How the pilot was awoken was not revealed by the ATSB report

How the pilot was awoken was not revealed by the ATSB report The pilot was flying from Devonport and overshot the destination by 46 kilometres

The pilot was flying from Devonport and overshot the destination by 46 kilometres An aviation expert says the rules governing pilot fatigue in Australia are "very acute"

The Australian Transport Safety Bureau (ATSB) is investigating the Vortex Air freight flight from the Tasmanian city of Devonport to King Island on November 8.

"During the cruise, the pilot fell asleep, resulting in the aircraft overflying King Island by 46 kilometres," the ATSB said.

The pilot of the Piper PA-31 aircraft was the only person on board when it missed the King Island airport about 7:15am.

Chairman of airline consultancy Strategic Aviation Solutions Neil Hansford said the aircraft would have been on autopilot.

"The aircraft will fly until it runs out of fuel, or in some cases the fuel tanks have to be switched over, so once it's starved of fuel it then crashes," Mr Hansford said.

"[The pilot] may have been carrying return fuel to go back to Devonport or to somewhere else.

"That would have put [the pilot] potentially up over the Victorian coast if he hadn't woken up."

Map The plane was flying from Devonport to King Island

The twin-engined aircraft involved in the incident has a cruising speed of around 380 kilometres per hour and a range of about 1,900km.

The plane has a passenger capacity of five to seven people, but was operating in its freight configuration.

The ABC has contacted the managing director of Vortex Air, Colin Tucker, for comment.

The ATSB would not say how it became aware of the incident, or how the pilot was woken up.

The ABC understands a pilot flying from Devonport to King Island may be contactable by either Melbourne or Devonport air traffic controllers, depending on whether they had manually switched frequencies, but there is no air traffic control on King Island.

Aviation fatigue rules very strict: expert

King Island lies in Bass Strait, north-west of Tasmania. ( ABC News: Simon Cullen )

Mr Hansford, who did not have specific knowledge of the incident, said it would be unusual for a pilot to fall asleep on a relatively short flight of about 240 kilometres.

"The fatigue rules in Australia are very, very acute," he said.

"There's no way in the world that someone should've taken on that flight fatigued."

He said he was aware of another pilot who had deliberately taken a nap heading towards Rockhampton in Queensland, with an alarm clock set, but forgot he would need to manually switch fuel tanks during the flight.

Vortex Air is a 25-year-old Australian airline that flies both freight and passengers to Tasmania, specialising in charter flights.

The ATSB is expecting its investigation to be completed in the first quarter of next year.