Thanks for your prompt replies Phoenix.
Hm, I'm at home now (AU TZ) using firefox on linux and I have AU date formats displaying - unlike earlier at work!
Perhaps there is one more piece to the puzzle?
At work previously, after making changes to the properties files aforementioned, I did a zmcontrol stop and zmcontrol start cycle on the zimbra server and reloaded the interface in several client browsers - firefox on windows and safari on mac. They retained the US date formats at that time.
One explanation I can find for this is that perhaps I had to flush the browsers cache in addition to reloading the AJAX interface?
And, yes, I have voted for the changes ;-)