MELBOURNE, July 21 -- The man responsible for overseeing the exhaustive search for missing flight MH370 remains positive about the prospect of finding the jet, even though the operation marked its 500-day milestone on Tuesday.

Australian Transport Safety Bureau (ATSB) chief commissioner Martin Dolan said spirits among the search party remained high, 16 months after the plane's unexplained disappearance on March 8 last year.

"We still remain confident it will be found in the next year," Dolan told News Limited on Tuesday.

"The enthusiasm and high morale (of the search crew) remains remarkable. We have a very impressive group of people working for us."

Flight MH370 is believed to have ditched into the southern Indian Ocean, some 2,000 km west of Perth, well outside of the plane's original flight path from Kuala Lumpur to Beijing.

So far, two search vessels have scoured around half of the 120, 000 square kilometer search area at the basin of the ocean floor.

There were 239 passengers on board the Boeing 777 when it mysteriously vanished, with the vast majority of those - 153 - being Chinese citizens.

The commissioner said it was unknown what would happen if Australian government's search effort - which reportedly could cost up to 73 million U.S dollars - failed.

"It's hard to speculate on what might happen if we're not successful," Dolan said. "All the information we have puts the aircraft in this defined search area."

Dolan hoped to add a third vessel to speed up the operation, once the winter weather dissipated around the Indian Ocean.

"If we have to search the entire area, it will be completed this time next year but we expect to find the aircraft before then, " he said.