Malaysia Airlines flight MH370 was almost certainly on autopilot when it ran out of fuel and crashed, with the crew likely "unresponsive", Australian officials said Thursday, announcing the search for wreckage would shift further south.

Investigators have been grappling with the mystery of the Boeing 777's disappearance on March 8 with 239 people on board, spending months scouring the Indian Ocean in vain.

An expert group has reviewed all the existing information and Australian Deputy Prime Minister Warren Truss said it was now "highly, highly likely that the aircraft was on autopilot" when it went down.

"Otherwise it could not have followed the orderly path that has been identified through the satellite sightings," he told reporters.

Martin Dolan, commissioner of the Australian Transport Safety Bureau (ATSB), which is leading the search, agreed.

"Certainly for its path across the Indian Ocean we are confident that the aircraft was operating on autopilot until it went out of fuel," he said.

He quantified this by saying the experts assessed that the plane flew in a straight line, according to the electronic "handshakes" it periodically exchanged with satellites.

"If you look at our detailed report, you will see there are seven arcs that we are looking at and we're saying the path the aircraft took to intercept each of those arcs was a straight path," he said.

The plane was flying from Kuala Lumpur to Beijing when it veered off course and vanished, shattering families of those aboard, who still have no idea what happened to their loved ones.

No trace of the plane has been found despite an extensive Australian-led search deep in the Indian Ocean, where Malaysia believes it crashed.

Theories on what happened include a hijacking, rogue pilot action or mechanical failure.

But a 64-page report into the disappearance by the ATSB released Thursday said the most likely scenario was the pilots and crew suffered from hypoxia, or lack of oxygen, where a plane loses air pressure at high altitude.

Hypoxia can cause confusion, hallucinations, poor judgement and eventually unconsciousness and death.

ATSB said "the final stages of the unresponsive crew/hypoxia event type appeared to best fit the available evidence for the final period of MH370's flight when it was heading in a generally southerly direction".

- 'Painstaking' search -

A review of the data has now identified a new area, covering up to 60,000 square kilometres in the southern Indian Ocean, where an underwater search will start in August and take up to 12 months.

"Specialists have analysed satellite communications information -- information which was never initially intended to have the capability to track an aircraft -- and performed extremely complex calculations," Truss said.

"The new priority area is still focused on the seventh arc where the aircraft last communicated with satellites.

"We are now shifting our attention to an area further south along the arc based on these calculations."

The new area is around 1,800 kilometres (1,100 miles) west of Perth and had previously been subject to an aerial search, which found no debris.

Truss said he was "optimistic" that "this site is the best available and most likely place where the aircraft is resting".

Until now, the most intensive search had been with a mini-submarine in a zone further north, where "pings" believed to be from the plane's black box flight recorders were detected.

That area has now been ruled out as the final resting place of MH370. The source of the noises is unknown.

Before the new underwater search can begin, the ocean floor, which is up to five kilometres deep, needs to be mapped.

Two ships -- Fugro Equator and Zhu Kezhen -- are currently surveying the area before a contractor begins the intensive undersea probe.

Truss said Australia was currently tendering for a company to take control of the search, while Malaysia was set to hire additional equipment such as a towed side-scan sonar, a multi-beam echo sounder and a sub bottom profiler for the underwater survey.