http://au.news.yahoo.com/world/a/21824683/the-coin-discovery-could-rewrite-history/ Beat up story but great detecting find in any case.
I know of someone who dug up a Roman coin from road tailings in Georgia, USA. Of course, no-one would think that the Romans had a colony in the USA. In this case no-one should really think that the English reached the New World that early either. I've been told by a historian that in the 1600's and 1700's that people used ANYTHING for money, including Roman coins. And if they were using Roman coins why wouldn't they be using a 16th century shilling? After all it was only a couple of hundreds of years old then and the silver was still good.
Roman coins have been found in oz also, does not mean a roman lost it, just that someone did in the last 2000 years. Much like many of the old chinese coins found on the australian gold fields.
