Authorities were racing the clock today to find the “black box” of the missing Flight MH370 before its signal goes silent, as Malaysia admitted it got the last words from the cockpit of the doomed plane wrong.

Australian vessel Ocean Shield, fitted with a US-supplied black box detector known as a “towed pinger locator” left Perth yesterday but is expected to take up to three days to reach the search zone in the remote southern Indian Ocean.

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.

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 multi-national 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.

If floating MH370 debris is eventually found, authorities plan to analyse the 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.

Ten planes and nine ships resumed the hunt today, 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.

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.

“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.”

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