Seen this a few days ago and thought it was cheep but haven't been paying attention on what the going price is of this game on PC.
Until I saw it elsewhere for around $56 so that was about a $10 saving. Then I looked up the price comparisons and saw that cdkeydeals has it for $42.99 but that is an English only version and the page won't load so I'm unsure if it's still available or even got the NukeTown maps.
If you go by the $70.49 from steam it's an even more impressive saving. Plus you get the gamer points from OzGameShop that you can eventually put towards something else or use some towards this if you have them laying around :)
OzGameShop is reputable, reliable and fast at delivering keys in my experiences, although I have noticed they have gotten a little more on the expensive side in the past few months across the board on pricing, they do still have the occasional bargin.
Unsure of the end date of this price, but on the corner of the deal it does say 'Special Offer' implying it's for a limited time.