score:5
In principle, when presented with an Australian passport, border guards are supposed to look for an entry stamp to check you haven't stayed too long in the Schengen area.
In practice, they do not always check that very carefully and even if they do, I don't see how you would risk anything more than a lecture since, as an EU citizen your presence was legal all along. If I were a border guard, I would probably be annoyed, though.