Unless of course Alisdair, Melbournians imported nails from the US just like they imported houses and other things. There is a very interesting and erudite chapter in Miles Lewis's on-line publication Australian Building a Cultural Investigation at http://www.arbld.unimelb.edu.au/~milesbl/dbmenu.html
which discusses nails (under metals).
Another point to consider is that US made ships if wrecked in Australia would of course have American fastenings.
I have a screw in my foot that looks like it came out of one of those hardware catalogues.
yours
Iain Stuart
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