It would have been a big secret had Oz been approached about its use back in 1936 - at that time, Japan expressed concern to the Oz govt that it had let some Pacific islands (Brit territory but administered by Oz) be used by the USA to built military airbases which Japan thought could threaten their trade routes - at the time, Japan was building a flourishing trade empire in the Pacific - the Americans regarded the Pacific as their "new frontier".
So the date of its use, or preparation, is rather important.