The air search for the missing Malaysia Airlines jet in the southern Indian Ocean has been called off, with any wreckage likely to have sunk.
Prime Minister Tony Abbott has declared a new, underwater phase will begin in the baffling and frustrating search for MH370.
"By this stage, 52 days into the search, most material would have become waterlogged and sunk," Mr Abbott told reporters in Canberra on Monday.
Australia is heading the hunt for the Boeing 777, which disappeared on March 8 carrying 239 people and is believed to have crashed into the southern Indian Ocean.
In this new phase, private contractors will use technology known as "towed sonar" to scour the probable impact zone - an area of about 700km by 80km of the remote southern Indian Ocean. The cost will be at least $60 million.
Mr Abbott said the search "certainly is not ending".
He conceded there was a "terrible" possibility the plane may never be found.
"I want the families to know, I want the world to know, that Australia will not shirk its responsibilities in this area," he said.
Mr Abbott's language was less certain than two weeks ago when he told reporters in China: "We are confident that we know the position of the black box flight recorder to within some kilometres."
When questioned on Monday about his previous assertion, Mr Abbott said he was transparent and upfront at all times.
He admitted there was a degree of frustration that no clues have been found as yet despite pings believed to have come from the plane's black box.
"We are still baffled and disappointed that we haven't been able to find undersea wreckage based on those detections," Mr Abbott said.
A multi-nation fleet of aircraft and ships has scanned more than 4.5 million square kilometres of remote ocean off Western Australia for 41 days with no signs of wreckage from the plane.
A Blue-fin submersible has been scouring a 400 sq km zone centred on what was thought to be a transmission from the black box before its battery died.
Former defence force chief and search coordinator Angus Houston says the new phase could take at least eight months, depending on the weather and how well the towed sonar equipment works.
The technology was used to find the Titanic in 3.8km-deep waters in the Atlantic Ocean in 1985.
The same system was used in 2008 to find HMAS Sydney, which sank off Western Australia in a sea battle in 1941.
Most of the ocean in the MH370 search area is between 4000 and 4500 metres deep.
Meanwhile, an initial report into the disaster is expected to be released by United Nations air safety authorities this week.
It is expected to call for real-time tracking of commercial aircraft - the same advice that was given after a 2009 Air France crash in the Atlantic Ocean. It took about two years to find that plane.
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