The Australian head of the search for missing flight MH370 believes it’s very likely the plane will be found, and it should be in good condition despite being underwater for 10 months.

“Our satellite calculations gave us an area we determined was high priority,” ATSB chief commissioner Martin Dolan says.

“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.”

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Mr Dolan says the aircraft is believed to be 4000 metres below the sea.

“There’s very little going on there… that’s likely to affect the components of the aircraft we are looking for,” he told AFP.

“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.

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

A fourth search ship with enhanced search capabilities, the Fugro Supporter, will join the operation later this month.

“(It) can be programmed and cover areas much more thoroughly. It’s of course a lot slower (but) we need to go slow so that we can be 100 per cent sure that we have covered that area totally,” Mr Dolan said.

Three vessels are currently probing the depths of the Indian Ocean off western Australia where the Malaysian Airlines plane carrying 239 people 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.

“We are confident that if we are looking in the right area – as we think we are – we will find the aircraft,” Mr Dolan said.

“It’s just that it’s a very large area, so it’s going to take a long time.”