MH370 Search co-ordinators have revealed the missing plane could have literally slipped through the cracks in the southern Indian Ocean floor - and crews are retracing their steps to be sure.

As the unexplained disappearance of the Boeing 777 approaches its two-year anniversary on March 8, the Australian Transport Safety Bureau has confirmed the possibility the plane may have been missed in areas already searched.

Chief Commissioner Martin Dolan said challenging terrain and deep fissures in the sea bed had meant they were not always able to get a "good enough sonar image".

"We're taking another look because the areas where we haven't been certain are large enough to contain an aircraft - which is why we're going over them," said Commissioner Dolan.

"The sea floor is very rugged and complex."

The hazards involved in the search have been highlighted in recent weeks by damage to the expensive equipment including a lost US$1 million tow fish, and a broken communications pole.

Commissioner Dolan said there was still an area "two-thirds the size of Tasmania" to search, and hopes remained high the Boeing 777 would be found before the operation was completed.

"There was never a guarantee of success, but we still think there's a high likelihood of success before we finish (in June)," he said.

"I still wake up every day thinking today could be the day."

Families were being kept well informed about the search progress and knew there was a possibility the operation could end in June with nothing found.

Brisbane's Jeanette Maguire, whose sister and brother-in-law Cathy and Bob Lawton were among six Australians on the plane, said it was difficult to face the prospect of never knowing what happened.

"I don't know how we're going to deal with that," said Ms Maguire.

"It's something we do talk about a lot together. We have complete confidence in the search being undertaken but you do have to be realistic."

Joint Agency Coordination Centre chief Judith Zielke said they had been preparing families since last April of the intention not to expand the search beyond the current 120,000 square kilometre area.

"No matter what we do that's an extremely difficult thing for the families to come to terms with and we will continue to hope that we're successful (in the search)," said Ms Zielke.

A Chinese vessel will set out for Australia on Sunday January 31 to join the search bringing to four the number of ships involved.

Commissioner Dolan said the ATSB was still awaiting the outcome of a French investigation into the flaperon confirmed as coming from MH370.

"At this stage we've not heard anything from the French that would enable us to form a view about what position the flaperon was in when it separated from the aircraft which is the key question for us," he said. "It's quite possible we won't get anything definitive on that."

Malaysia is also due to release another report on the anniversary of the plane's disappearance, en route from Kuala Lumpur to Beijing.

"We're still waiting to understand what it is that they're intending to publish", said Mr Dolan.

"That's up to them."

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