The rarest Australian coin is this gold half sovreign struck in 1888 at the Perth mint.Here is the story which is 3 years old,so I imagine you could add a bit to the $125.000 price they're talking about.At the bottom of the article,they mention a 1930 proof penny that was worth $281,750,so it's the most expensive.Only US coins have broken the million dollar barrier (I think) but I would imagine if this one-of-a-kind British gold dark-ages penny ever goes to auction again,it will go way over.It was sold for just over $411.000,to an American..hehe
http://www.spink.com/resources/press/4018penny_pr.asp http://www.theage.com.au/articles/20...354595518.html 
