Missing airliner MH370 is "very likely" to be found if it lies in the vast undersea zone now being scoured, and is probably in good condition despite being submerged for 10 months, the Australian search chief told AFP.

Three vessels, with a fourth on the way, are probing the depths of the Indian Ocean off western Australia where the Malaysian Airlines plane carrying 239 people, mostly Chinese, is believed to have crashed.

The jet disappeared en route from Kuala Lumpur to Beijing on March 8 last year, and apart from some mysterious satellite "pings" believed to plot its southern course, no sign of it has been found despite a massive air and sea operation.

Relatives of those on board have endured a long wait for answers on what happened to their loved ones, with their torment reawakened by AirAsia Flight QZ8501 crashing into the sea off Indonesia on December 28.

So far, one quarter of the priority underwater search area of 60,000 square kilometres (23,166 square miles) has been checked, while a wider zone of 208,000 square kilometres has been mapped.

"Our satellite calculations gave us an area we determined was high priority," Martin Dolan, the chief commissioner of the Australian Transport Safety Bureau, which is leading the search, told AFP.

"In this 60,000 square kilometres, it's very likely we will find the aircraft, but we don't know exactly where. We just have to cover that area thoroughly until we find the aircraft."

The priority search began in early October and will accelerate over the next few months as weather conditions improve, with the hunt expected to wrap up in May.

If the jet is not found, a decision on extending the investigation would be made by Australia and Malaysia, which have jointly shouldered the cost.

Undersea revelations

Dolan said mapping had led to the discovery of previously unknown undersea features such as mountains, volcanoes, chasms and a rough, uneven sea floor, highlighting the challenges.

To take a closer look at the complex terrain, the Australian and Malaysian governments said Wednesday they were jointly funding the fourth ship, Fugro Supporter, to join the probe later this month.

While the other three vessels -- Fugro Equator, Fugro Discovery and GO Phoenix -- use sophisticated sonar systems attached to tow cables up to 10 kilometres (6.2 miles) long, the Supporter will have an autonomous underwater vehicle.

"(It) can be programmed and cover areas much more thoroughly. It's of course a lot slower," Dolan said, adding that about five percent of the search area needed the closer scrutiny.

"We need to go slow so that we can be 100 percent sure that we have covered that area totally."

No oxygen, no decay

The underwater probe is taking place in treacherous surface conditions with waves as high as 12 metres (39 feet). Authorities believe the plane may be sitting on the ocean floor at depths of 4,000-metres.

But the deep sea plays an important role in preserving the aircraft if that was its resting place, Dolan said.

"At the likely depth we think the aircraft is, around about 4,000 metres below the sea surface, there's very little going on there... that's likely to affect the components of the aircraft we are looking for," he said.

"The second is that down there there's very little or no oxygen, so there's not anything in the way of oxidisation or decay going on with aircraft parts.

"The sonar equipment we are using means that -- even if there's an amount of silt or other things -- we can still locate the aircraft parts.

"We are confident there's going to be enough visible parts of the aircraft for us to be able to detect it."

Authorities have drawn up a recovery plan if the plane is found.

The proposal, which is still being assessed, would use technology and techniques similar to those deployed to recover Air France flight AF447, Dolan said.

The French flag carrier, including its "black box" flight data recorder, was hauled from the Atlantic nearly two years after it crashed en route from Rio de Janeiro to Paris on June 1, 2009, killing all 228 on board.

Some Aus$120 million (US$98 million) has so far been jointly committed by Australia and Malaysia to fund the search.

"We are confident that if we are looking in the right area -- as we think we are -- we will find the aircraft," Dolan said.

"It's just that it's a very large area, so it's going to take a long time."