The head of the Australian agency leading the hunt for Flight MH370 says he is confident the plane will soon be found.

Mr Martin Dolan, head of the Australian Transport Safety Bureau, said the multinational search effort still had a large area of rugged ocean floor off Western Australia to search by a June deadline.

Asked if his hopes were dwindling as the remaining search zone becomes ever smaller, Mr Dolan said: "No. There is an increased likelihood that the plane will be found in the remaining area. We still have over 30,000 sq km to go - an area half the size of Tasmania.

"We remain optimistic," he said in an interview with The Sunday Times, days before the second anniversary of the March 8 disappearance of the Malaysia Airlines Boeing 777 with 239 people on board.

The painstaking search, which now involves four ships, has covered about 87,000 sq km of the targeted zone in the southern Indian Ocean, west of Perth.

Hopes of a breakthrough were lifted last week when a piece of debris that could be part of the plane was found in Mozambique .

But Mr Dolan said it is too early to tell if the wing fragment would shed light on the search. The debris was to be sent to Australia.

The search is believed to be the most complex and difficult in aviation history.

It involves ocean depths of 3,500m to 6,000m across an uneven sea floor which includes mountains, cliffs, ravines and inactive volcanoes.

The area is remote and uncharted, and a lengthy mapping operation was required before the underwater search began.

Search crews need five or six days to reach the area from their port in Western Australia and, as Mr Dolan said, "the weather is regularly bad, and sometimes awful".

The total cost, if the entire search area is covered, will come in at A$180 million (S$184 million), with A$100 million provided by Malaysia, A$60 million by Australia and A$20 million by China. Mr Dolan said the discovery of two shipwrecks had given him confidence the hunt was functioning well and was "not likely to be missing anything out there".

He said the agency was working on the assumption that the plane had no pilot intervention during the latter stages of its flight and ran out of fuel.

However, Mr Dolan said failure to find the plane in the current search zone would challenge that theory and indicate an individual was "deliberately" piloting the aircraft at the end of the flight.

"The range of the evidence makes it by far the more likely scenario that no one was making control inputs to that aircraft at the latter stages of the flight," he said.

"If we don't find the plane in the current search area, that is pretty clear evidence that one of the other scenarios is more likely."

Mr Dolan said the other most likely possibility was that the plane was deliberately glided after it ran out of fuel, presumably by a so-called rogue pilot.

No evidence has been found to implicate any of the passengers or crew aboard the plane, which mysteriously veered off course and disappeared from radar on a flight from Kuala Lumpur to Beijing in 2014.

"There are three possible end-of-flight scenarios," Mr Dolan said. The most likely was the plane was on autopilot.

"The second is that some individual deliberately controlled the flight of the aircraft even after it lost power, and so glided the aircraft. The third - which is the least likely - is a complicated sequence which would have the aircraft being ditched under power.

"It is very unlikely," Mr Dolan said. "If someone had been actively flying that aircraft on its journey across the Indian Ocean, then it would have gotten further than it did," he said.

"To conserve fuel, he would take step climbs and change the (altitude) of the aircraft as fuel is consumed.

"The variety of possible flight paths shows a straight course consistent with an uninterrupted autopilot operation."

He said he had not been asked to assess other potential search areas and there were no plans to continue the search beyond June.

Any new alternative zone would be "impossibly" large, he said, because the authorities would have to assume the plane was manually glided to its resting place. This assumption would triple the size of the current search zone.

"The governments who are funding the search have made it clear that once we have completed this search area, that is it," Mr Dolan said.

"We are satisfied that in the areas we have searched, we have been able to give 100 per cent reassurance that the aircraft is not there," he added.

"We remain very confident that we are searching in by far the highest probable area and that if the plane is there, we will find it."

He said: "I hope that the next conversation we have is about how we found the aircraft."