Australian vessel Ocean Shield, fitted with a US-supplied black box detector known as a "towed pinger locator" left Perth on Monday but is expected to take up to three days to reach the search zone in the remote southern Indian Ocean. (Black box detector to join search for missing Malaysian flight MH370)
A black box signal usually lasts only about 30 days and fears are mounting that time will run out, after the Malaysian Airlines plane carrying 239 people veered off course and vanished on March 8. (Missing plane's black box batteries will die by mid-April)
Australian Defence Minister David Johnston admitted there was only a slim chance it would be found as debris needs to be positively identified first to nail down a crash site.
"We've got about a week (left), but it depends on the temperature of the water and water depth and pressure as to how long the battery power will last," he told national radio.
Authorities are scouring a massive expanse of ocean for clues and even if the zone is narrowed down, Ocean Shield must tow the equipment at just five kilometres per hour (3.1 mph) for the pinger to be able to pick up a signal.
Despite a far-reaching multinational search of the vast and desolate seas nothing has yet been identified, with repeated sightings of objects turning out to be fishing gear or flotsam. (Debris retrieved from Indian Ocean not from MH370: report)
If floating MH370 debris is eventually found, authorities plan to analyse recent weather patterns and ocean currents to determine where the plane went down.
Malaysia believes the flight, en route from Kuala Lumpur to Beijing, was deliberately diverted by someone on board and that satellite data indicates it crashed in the Indian Ocean. (Military data suggests 'skilled' flyer turned Malaysia Airlines jet)
Last words spoken
Ten planes and nine ships resumed the hunt on Tuesday, hours after Malaysia's civil aviation department said the last words spoken by one of the pilots were "Good night Malaysian three seven zero".
The phrasing was different to the more casual "All right, good night" originally reported. ('Good night': Haunting final contact from missing Malaysian Airlines jet)
Malayasia's handling of the crisis has been widely criticised, with Chinese relatives of those on board the missing plane particularly scathing, accusing it of incompetence and even a cover-up. (Malaysia's jet crash announcement draws criticism)
"We would like to confirm that the last conversation in the transcript between the air traffic controller and the cockpit is at 0119 (Malaysian time) and is 'Good night Malaysian three seven zero'," the aviation department said in a statement.
"The authorities are still doing forensic investigation to determine whether those last words from the cockpit were by the pilot or the co-pilot."
Malaysia Airlines chief executive Ahmad Jauhari Yahya had said on March 17 that the last words from the cockpit were believed to have been spoken by the co-pilot.
Shortly after the final message communications were cut and the Boeing 777, carrying mostly Chinese nationals, vanished from civilian radar.
'We are not hiding anything'
The move came after testy exchanges on Monday between foreign journalists and Malaysian Transport and Defence Minister Hishammuddin Hussein, who insisted: "We are not hiding anything, we are just following the procedure that has been set."
Malaysia insists it is being transparent, but has yet to release any details of its investigation into what happened, which has included probing the backgrounds of everyone on the flight, including its crew.
In the early days of their daily press briefings after the plane went missing, Malaysian officials made a series of contradictory statements that added to the confusion.
Notably, there have been about-turns regarding the crucial sequence of events in the plane's cockpit before it veered off course, and Malaysia's armed forces have been criticised for failing to intercept the diverted plane when it appeared on military radar.
While Malaysia remains officially in charge, Australia has assumed increasing responsibility for the search, appointing retired air chief marshal Angus Houston to head a new coordination centre in Perth.
Malaysian Prime Minister Najib Razak is expected in Perth on Wednesday to tour the air base being used a staging post.