Hi J5,
Actually the Federal Police are quite accustomed to minor SNAFUs like this happening. They themselves sometimes make them too. For instance when a fully loaded flight arrives late at night in many airports and they're understaffed they often are in such a rush to process everybody that they sometimes neglect to stamp a passport, it happens. They take these things in stride and the mere fact that you've returned to resolve the situation shows them that you're one of the good guys!
That usually scores big points with them.
Cheers,
James Expat-blog Experts Team