SPECIMEN overprinted stamps used as postage...

I've seen overprinted "Specimen" stamps used on cover before (I will dig out my other one or two) but thought surely the postal authorities would catch this illegal $10 usage.  It was on a larger cover used to mail a friend some winning online auction lots, and he already cut the stamps out.  Too bad, as I would have been happy to shame the seller for defrauding Australia Post!  For what it's worth, I seek out unusual covers like this ("entires" rather than just the cut-out stamps) so if anyone has any please email or private message me and I would definitely be interested!