Australia has announced that the ADV Ocean Shield has detected signals in the Indian Ocean, consistent with that emitted from a black box, in its search for the missing Malaysia Airlines Flight MH370.

Joint Agency Coordination Centre (JACC) chief Angus Houston said the signals were picked up twice in the last 24 hours.

On the second time, it was determined the signal likely came from a black box, Houston added.

"On this occasion, two distant pings returned were audible. Significantly, this would be consistent transmission from both the data flight recorder and the cockpit voice recorder," he told a press conference in Perth, Australia, today.

Houston said ADV Ocean Shield picked up the signal for two hours and 20 minutes before losing contact and making a turn around to reacquire the signal.

"The second detection on the return round was held for approximately 13 minutes," he said.

Houston described this development as the "most promising lead" and "best information" so far.

He said ADV Ocean Shield was now attempting to reacquire the signal and narrow down its location before deploying an autonomous undersea vehicle into the ocean.

Joint Taskforce 658 commander Peter Leavy told the same press conference that ADV Ocean Shield needs to detect the signal just once more before it sends the AUV to map the seafloor with its sonar and a camera.

He warned that sound does not necessarily travel in a straight line in water, and can behave in ways that are hard to predict depending on the water pressure, salinity and temperature, and can even be bent 90 degrees by these conditions.

In addition, ADV Ocean Shield can only move slowly - about three knots (5.6 km/h) - with its towed pinger locator deployed and would take several hours just to make a turn and make another sweep.

“We have the best in the world out there doing it though, so we are confident that if there is a pinger out there and it is still radiating, we should find it,” he said.

Houston explained that MH370’s incomplete ‘handshake’ with an Inmarsat’s communications satellite had occurred when the aircraft had just run out of fuel.

This handshake took place at 8.19am , about eight minutes after the sixth and final complete handshake with the satellite at 8.11am .

“This experts consider this being very significant. They think something happened at that stage, and we assessed that that is about where the aircraft would have run out of fuel,” he said.

The current underwater search area is estimated based on satellite data and the aircraft’s performance, with ADV Ocean Shield searching the northern part of the area, and the Chinese patrol vessel Haixun 01 and British survey vessel HMS Echo investigating another possible ping at the southern end.

The size of the area is due to different assumptions made regarding the aircraft’s actual speed, he said, while the ongoing surface search are in different areas because it needed to take account of 30 days of drifting in the sea.

“All of the search underwater is being enabled by that wonderful work that was done by the expert team in Kuala Lumpur, and their work has enabled us to come up with an underwater search area that is quite narrowly focused.

“With the acoustic events that we are getting in the area, we are encouraged that we are very close to where we need to be."

Nothing certain as yet

However, Houston cautioned that until wreckage of the aircraft is found, nothing is final.

"Again I would ask all of you to treat this information cautiously and responsibly until such time we can provide an unequivocal determination.

"We haven't found the aircraft yet and we need further confirmation," Houston said, adding this could take days.

He cautioned that, even if the aircraft is found, a salvage operation could take months to complete.

“In very deep oceanic water, nothing happens fast. Of course, I will update you once we have an unequivocal determination. Ocean Shield will stay in the area until such time that it can verify or discount the detections,” he said.

He also acknowledged the uncertainty as the black box's battery life has exceeded its guaranteed life span and could die at any time, although he expressed the hope it could last a few days more.

The search operation for Flight MH370 entered day 31 today as a multinational effort is scouring the Indian Ocean approximately 1,700km northwest of Perth.

The Beijing-bound Boeing 777-200ER aircraft went missing not long after taking off from KL International Airport in the early hours of March 8, with 12 crew members and 227 passengers.

Investigators have determined the aircraft deliberately cut communications with air traffic tower controllers and diverted the plane on an unknown trajectory.

However, based on the aircraft's endurance and satellite data, investigators have determined that its last location to be over the Indian Ocean.