UPDATE: Australia's former defence chief Angus Houston says the Malaysia Airlines search and recovery operation is the most challenging he has ever seen.
Air Chief Marshal Houston is leading the new Joint Agency Coordination Centre, which will have about 20 staff and be headquartered in West Perth at Dumas House near Parliament House.
"We will continue pursuing the search with much vigour," he told reporters in Perth.
"I have to say in my experience - and I have got a lot of experience in search and rescue over the years - this search and recovery operation is probably the most challenging I have ever seen."
He said the search "could drag on for a long time".
"We've been searching for many, many days and so far have not found anything connected with MH370," he said.
A fourth Australian ship, the Seahorse Standard, left today to assist with surface sweep operations and would take five days to reach the search zone, the Department of Defence said.
Commodore Peter Leavy said search vessels were experiencing strong winds and heavy seas but conditions were expected to ease.
“The priority remains to find objects that can be linked to missing Malaysia Airlines flight MH370,” he said in a statement.
Air Chief Marshal Houston was asked about Danica Weeks, the wife of missing passenger Paul Weeks, after she visited the RAAF Pearce air base north of Perth on Tuesday.
He said he had passed on his personal phone number to her and urged her to go to the centre for a one-on-one briefing.
Malaysian Prime Minister Najib Razak will visit Perth on Wednesday and Thursday to tour Pearce and personally thank the international forces taking part in the search.
The international search for missing Malaysia Airlines flight MH370 is in a race against time, with only about a week of battery life remaining on the plane's black box recorder.
Defence Minister David Johnston said today it would take two or three days for the Australian naval vessel Ocean Shield, which has been fitted with a pinger locator, to reach the search zone.
The satellite “pinger” within the missing plane's black box has about 30 days of battery life.
“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,” Senator Johnston told ABC radio.
Ten planes and nine ships will assist in today's search, the Joint Agency Coordination Centre said.
Military aircraft from Australia, Malaysia, China, the US, Japan, South Korea and New Zealand will take to the skies.
Weather in the search area is expected to be poor with areas of low visibility.
Overnight Malaysia's civil aviation authority revealed that the last words spoken by one of the pilots of MH370 were "Good night Malaysian three seven zero", changing the previous account of the last message as a more casual "All right, good night".
The correction of the official account of the last words was made as Malaysian authorities face heavy criticism for their handling of the disappearance, particularly from families of the Chinese passengers on board Flight MH370 who have accused Malaysia of mismanaging the search and holding back information.
"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 Department of Civil Aviation said.
Malaysia's ambassador to China told Chinese families in Beijing as early as March 12, four days after the flight went missing, that the last words had been "All right, good night".
"Good night Malaysian three seven zero" would be a more formal, standard sign-off from the cockpit of the Boeing 777, which was just leaving Malaysia-controlled air space on its route from Kuala Lumpur to Beijing.
Minutes later its communications were cut off and it turned back across Malaysia and headed towards the Indian Ocean. More than three weeks later, a huge international search effort is going on in the southern Indian Ocean off Western Australia, but has so far failed to turn up any wreckage.
The statement from the civil aviation authority came after Malaysia's acting Transport Minister Hishammuddin Hussein was questioned at a news conference overnight over the last words from the cockpit and fended off demands to release the official transcript.
The statement said authorities were still conducting "forensic investigation" to determine whether the last words from the cockpit were by the pilot or the co-pilot. Previously, Malaysia Airlines said that the words were believed to have come from the co-pilot.
The civil aviation department said the investigating team had been instructed to release the full transcript at the next briefing with the next of kin.
Malaysia says the plane, which disappeared less than an hour into its flight, was likely to have been diverted deliberately far off course. Investigators have determined no apparent motive or other red flags among the 227 passengers or the 12 crew. About two-thirds of the passengers were Chinese nationals.
Planes, ships in seven-nation MH370 search
Yesterday orange objects spotted by a plane searching for the missing Malaysia Airlines jet turned out to be nothing more than fishing equipment.
Despite the false alarm, in Perth yesterday Prime Minister Tony Abbott said the search would not be scaled down.
"I'm certainly not putting a time limit on it ... we can keep searching for quite some time to come," Mr Abbott told reporters on Monday at RAAF Pearce, the Perth military base co-ordinating the operation.
"We owe it to the families, we owe it to everyone that travels by air, we owe it to the anxious governments of the countries who had people on that aircraft. We owe it to the wider world which has been transfixed by this mystery for three weeks now."
Senator Johnston said about 1000 sailors were looking for debris at sea - but the task was still onerous.
While each country involved was currently bearing its own costs, Australia was paying for running the co-ordination centre, which will have about 20 staff and be led by retired air chief marshall Angus Houston from Perth CBD headquarters.
Mr Abbott also said his Malaysian counterpart was not too hasty in announcing last week - before any debris had been recovered or confirmed as being from MH370 - that the plane was lost in the southern Indian Ocean and all on board were assumed dead.
"That's the absolute overwhelming weight of evidence and I think that Prime Minister Najib Razak was perfectly entitled to come to that conclusion," he said.
In China, home to 153 people on board the flight, a comment piece in the China Daily newspaper called for "rationality" among relatives - some of whom insist their loved ones could still be alive.