@thommo101
The focused underwater search area is defined as a circle of 10km radius around the second Towed Pinger Locator detection which occurred on 8 April.
Source:-
http://jacc.gov.au/...
Contrary to what JACC has said (and repeated), the Bluefin-21 search area was centered on the #2 Ping detection of 05 April, i.e. 21°04'S 104°00'E.
As you have rightly pointed out, the #1 Ping detection was outside the 10km radius and in 4600m of water.