The hunt for the plane, spanning 60,000 square kilometres, is focusing on a narrow strip of water in the remote southern Indian Ocean and is due to end later this month.
No sign of wreckage of the Malaysian Airlines jet, which was carrying 239 people when it disappeared on March 8 last year en route from Kuala Lumpur to Beijing, has been found despite an intensive air and sea hunt.
"This will increase the search area by an additional 60,000 square kilometres to 120,000 square kilometres."
The extended hunt was first flagged by the Australian, Malaysian and Chinese governments last month. Most of the passengers were Chinese.
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Some Aus dollars 14.4 million has already been allocated this financial year to the agency leading the search, the Australian Transport Safety Bureau.
The government said the cost could be lowered by contributions from other countries.
Four specialist vessels are scouring the depths of the search zone off the west coast of Australia after the ocean floor was closely mapped last year.
The ships, including three from Dutch company Fugro, are carrying specialist equipment including towed sonar that can scan some 10 kilometres below the surface.