The Australian Defence Vessel Ocean Shield, towing a US pinger locator, detected two sets of pings, on Tuesday afternoon and then again about five hours later.

The signal detected on Tuesday afternoon was held for about five minutes and 32 seconds; followed by a second signal on Tuesday night, which was held for seven minutes.

The development came more than two days after Ocean Shield first detected the month-long search's most promising lead: two acoustic events on Saturday.

Retired air chief marshal Angus Houston, the head of the search's Joint Agency Co-ordination centre, announced the "promising" development during a press conference in Perth on Wednesday afternoon.

It revives hope the missing Malaysia Airlines plane's black box is still transmitting data days after its batteries were due to run dead.

Mr Houston said expert data analysis on previously detected signals had found they were not of natural origin, further boosting hopes of a breakthrough.

Mr Houston said experts believed the signals were consistent with those of a flight data recorder.

He said the first two pings - detected on April 5 at 4.45pm and at 9.27pm Perth time - had been analysed by the Australasian Joint Acoustic Analysis Centre, based at HMAS Albatross in Nowra, on the NSW south coast.

"The analysis determined that a very stable, distinct and clear signal was detected at 33.331 kilohertz, and that it consistently pulsed at a 1.106-second interval," Mr Houston said.

''They therefore asses that the transmission was not of natural origin, and was likely sourced from specific electronic equipment. They believe the signals to be consistent with the specification and description of a flight data recorder.''

He said two latest pings were detected on Tuesday - at 4.27pm and 10.17pm, Perth time.

Mr Houston said up to 11 military aircraft, four civil aircraft and up to 14 ships would assist in Wednesday's search, while the Ocean Shield would also conduct a sonar voice search.

The search area has been refined to a single zone, about 75,423 square kilometres in size, 2261 kilometres north west of Perth, which was a considerably smaller area than in previous days, he said.

''Based on Ocean Shield's detections, we are now searching a much more concentrated area, based on the drift predications made possible by Ocean Shield's detections,'' Mr Houston said.

''The smaller area has also allowed us to plan much tighter search patterns based entirely on visual search principles. In other words, we have intensified our search in the visual search area.

"There's no second chances," he said.

"It looks like the signals we picked up recently have been much weaker than the original signals we picked up ... we're either a long away from it or in my view more likely the batteries are starting to fade."

Although authorities have previously said the autonomous underwater vehicle Bluefin-21 would be dropped to the ocean floor once they received a third ping, they have yet to deploy the asset.

"By triangulating this data we will be able to come up with a much smaller search area under water," Mr Houston said.

"Time spent on the surface, we're covering six times more area than we would be able to do when we get under water. With the batteries due to fail shortly, we need to get as much positional data as we can."

An RAAF aircraft has been diverted to Ocean Shield to drop buoys around the field to lay a "sonar buoy pattern", which will involve underwater and floating components transmitting data back to the aircraft.

"That will provide a range of sensors, 1000 feet down, which is 1000 feet closer to the pinger locator on the ocean floor," Commodore Leavy, the operational head of the Australian search, said.

The Malaysia Airlines Boeing 777 vanished without a trace during en route to Beijing from Kuala Lumpur on March 8.

Despite an extensive and costly international search operation off the West Australian coast, the effort has failed to turn up a single piece of evidence to confirm the plane's final resting place.

SMH.COM.AU