I dunno what to think... it says it's region free, and has the original CD-key with it. Plus, the guy in the first link has very positive feedback, I'd imagine if the game didn't work on aussie systems then he'd have some negative feedback, which he doesn't.
Also, I thought if you bought off steam it was in american dollars, so $50 american is probably $56 Australian. Still I've seen retailers selling it for anywhere between $79-$100, just confusing why I see such a discrepancy between aussie shops, steam, and ebay.