Did you create the en-AU locale after the site had been running live with the en-US locale? Perhaps there are customers who originally visited the site with the en-US locale was the only one available and haven't been back to the site since the en-AU locale was added. You can confirm by running the following sql statement against your database
Code:
select count(*) from dbo.Customer where LocaleSetting='en-US'
If this returns anything but 0, then you have customers that still have en-US set as their default locale.
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