CANBERRA, Australia - A third ship will join the deep sea hunt for a missing Malaysian airliner as the 13-month-old search of a huge expanse of the Indian Ocean ramps up during the southern hemisphere summer, officials said Wednesday.

The search for Malaysia Airlines Flight 370 was scaled back to two ships towing sonar equipment during the winter when the remote target area more than 1,800 kilometres southwest of Australia was buffeted by gale-force winds and mountainous waves.

The search has covered more than 70,000 square kilometres since it began in October last year based on analysis of scant satellite information that tracked Flight 370's final flight on March 8 last year.

Authorities are unable to explain why the Boeing 777 went far off course with 239 people aboard during a flight from Kuala Lumpur, Malaysia, to Beijing.

A third ship Havila Harmony, equipped with a video camera inside an underwater drone, will leave the Australian port of Fremantle on Saturday and reach the remote search area five days later, the Australian Transport Safety Bureau said in a statement.

The bureau, which conducts the search on behalf of Malaysia, said the Havila Harmony would investigate rugged seabed terrain which cannot be effectively searched with less manoeuvrable towed side-scan sonar.

One of the two ships that made slow progress throughout the winter, the Fugro Discovery, began the 2,800-kilometre, six-day voyage back to Fremantle on Saturday because a crew member had become sick, the bureau said. The bureau gave no details of the crew member's condition.

The same ship returned to Fremantle earlier in November after only a day of searching because of a crew member's suspected appendicitis. The Fugro Discovery had managed only six days of searching the 120,000-square-kilometre priority area before it was forced back again for medical help. All ships have a doctor on board.

The ships are resupplied at Fremantle monthly and spend half their time at sea travelling to and from the search area.