The area the ships are searching was chosen based on hourly satellite signals the aircraft gave off after it vanished from radar on March 8 on a flight from Kuala Lumpur to Beijing.
It was carrying 239 people, including Perth-based NZ father of two Paul Weeks and Aucklander Ximin Wang.
Malaysia's Prime Minister, Najib Razak, who visited the headquarters of the multinational search in Perth yesterday, promised relatives of the passengers and crew that "we will not rest until answers are found".
Malaysian police have expanded an investigation into the missing plane to include ground crew, caterers and the farmers who picked four tonnes of mangosteens, a tropical fruit, that were in the hold.
Despite failing to find any suspicious evidence about the passengers or crew, Malaysian police chief Khalid Abu Bakar said he was confident the case would be solved.
He said police traced the mangosteens, a tangy Asian fruit with leathery skin around juicy white flesh, to an orchard in the city of Muar and examined both the farmers and the Chinese importers.
"Imagine how many people we must interview to rule out sabotage," the police chief said.
"We tracked down who plucked the fruits, who packed and shipped them out, and who put them on the plane. Then, we had to see who was to receive them in China and who paid for it, and how much."
Police have cleared the 227 passengers on board the flight, but are still investigating the 12 crew members.
Mr Khalid said police were examining all aspects of the flight including the caterers, who were investigated to ensure food served on the plane was not poisoned.
The Australian navy ship Ocean Shield, which is towing a US Navy signal locator, and the Royal Navy's HMS Echo, which has underwater search gear, will search a 240km track in a desolate patch of the southern Indian Ocean, said Angus Houston, the head of a joint agency co-ordinating the search.
"The area of highest probability as to where the aircraft might have entered the water is the area where the underwater search will commence," Mr Houston said.
"It's on the basis of data that only arrived very recently and it's the best data that is available."
Also helping in the hunt is a British nuclear-powered submarine, HMS Tireless, which has advanced underwater search capability.
Australian Prime Minister Tony Abbott said Australia was "throwing everything we have" at the search.
But he also warned that "we cannot be certain of ultimate success" - a contrast to previous statements in which he pledged that if there was wreckage to be found, Australia would find it.
Selamat Omar, whose 29-year-old son, Mohammed Khairul Amri Selamat, was on the plane, told Australia's ABC radio he was still hoping he was alive.
"After such a long time, they could not find any object [wreckage], even with the expertise of the helpers," he said.
"We are sure that there is hope of life."
