Interesting, I missed that post.
I doubt this coin is antique, cast celtic coins of that time are much smaller in diameter (look on google for "potin"), at least in Gaul/Germany/Brittain.
Philippe 2 of Macedonia coins were the model for celtic coins, but aall of them have an horse on one side, and some kind of portrait on the other side, which is not the case here.
As for the presence of Roman Celtic coins in Australia... well, I've lost some myself: I keep a few roman coins in my wallet, and I've lost a few.
I found them back in Europe, have quite a lot of those, they are worn out with little value, so if someone finds them, it will be fun!