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

Three vessels, with a fourth on the way, are probing the depths of the Indian Ocean off western Australia where the Malaysia 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 and apart from some mysterious satellite "pings" interpreted as plotting its southern course, no sign of it has been found despite a massive air and sea operation.