If you have a big sound receiver you can hear things that are a long way down.
There were rumors that the French submarine Emeraude that went looking for AF447 couldn't really exploit its listening equipment to the full because while it was large, it didn't have much sensitivity at such high frequencies.
Do the Australian navy have ships with suitable big sonar to hand? They seem to
Leeuwin-class survey vessel - Wikipedia, the free encyclopedia but on the wrong side of the country. Looks like there should be room for some extra stuff onboard if the US has a special 40+ KHz sensitive device.