A site about acoustic tags on marine animals says:
The simplest electronic tags are acoustic tags. These can be surgically implanted inside the animal, or attached to a tether that has a delayed remote release mechanism. Two types of acoustic tags are used on marine animals: coded tags and continuous pingers. Coded tags regularly emit an ultrasonic signal that encodes a unique identifying number. This signal is detected and logged when the tagged animal is in close range (a few hundred metres) of a receiver stationed on the seabed or attached to a mooring. Continuous pingers are used for real-time tracking of marine animals from vessels fitted with a receiver for detecting the pinger. Some acoustic tags measure and transmit depth and temperature data when the animal is in range of the receiver. Coded acoustic tags are used to determine when animals visit particular sites, and can be used to examine long range movements between areas. Their batteries can last up to 10 years, allowing researchers to see how the behaviour of animals changes during different phases of life. Acoustic tags have been used since the late 1990s to study how white sharks occupy and move between coastal areas around Australia.
Ocean Tracks | Acoustic Tags
I presume that the SAR investigators were able to rule out the possibility of coded tags, for lack of that unique identifying number? (And why doesn't an aviation ULB broadcast the same unique information?)
That leaves continuous pingers as a possibility. Presumably, marine researchers have records of what they've tagged and where those tags were last 'heard'? And presumably the MH370 investigators will have co-ordinated with those science agencies?
There has to be a reason for the Australian's certainty about the source of the pings. Doesn't there?