Australian search chiefs said Tuesday they now have a better understanding of where flight MH370 might be, admitting it was inconceivable that a commercial plane could vanish in the modern era.

The Malaysia Airlines jet with 239 people on board disappeared in March 2014 en route from Kuala Lumpur to Beijing, sparking a massive underwater search in the remote southern Indian Ocean which ended in January.

No trace of the aircraft was found in a 120,000 square kilometre (46,000 square mile) zone based on satellite analysis of the jet's likely trajectory after it diverted from its flight path.

"The reasons for the loss of MH370 cannot be established with certainty until the aircraft is found," the Australian Transport Safety Bureau (ATSB), which led the search mission, said in its final report Tuesday. 

"It is almost inconceivable and certainly societally unacceptable in the modern aviation era... for a large commercial aircraft to be missing and for the world not to know with certainty what became of the aircraft and those on board."

The hunt for the plane was the largest in history and the ATSB said the challenge was working with limited data, with only aircraft performance information and satellite communication metadata available initially.

Later during the underwater search, long-term drift studies were used to trace the origin of debris which by then had been floating for more than a year, and in some cases over two years. 

But after a near three-year hunt, it said in the 440-page report that the "understanding of where MH370 may be located is better now than it has ever been".

"The underwater search has eliminated most of the high probability areas yielded by reconstructing the aircraft’s flight path and the debris drift studies conducted in the past 12 months have identified the most likely area with increasing precision," it said. 

Australia's national science body CSIRO released a report in April suggesting the doomed plane was "most likely" north of the former search zone in an area of approximately 25,000 square kilometres.

The ATSB agreed, having re-analysed satellite imagery taken on March 23, 2014 which identified a range of objects which may have been MH370 debris.

"This analysis complements the findings of the First Principles Review and identifies an area of less than 25,000 square kilometres which has the highest likelihood of containing MH370," it said.

Only three fragments of MH370 have been found, washed up on western Indian Ocean shores, including a two-metre wing part known as a flaperon.

A US seabed exploration firm said in August it wanted to resume the search, with relatives of passengers aboard the flight calling on Malaysia to accept the offer.

The ATSB said the incident had led to some important lessons about locating missing aircraft.

"Requirements and systems for tracking aircraft have been enhanced and will continue to be enhanced," it said.

"Steps are being taken to advance other aircraft systems including emergency locator transponders and flight recorder locator beacons."