You bought the case for less than half the best price I can find it in the US (they may be a few dollars under the USD40 [AUD56] price, but only a few) - that alone says it's a knock-off. But it probably will protect the phone just as well, or almost as well, as the original. (Otterbox isn't showing an S5 case any more, but some stores may have a few, and Otterbox will sell you one if they have one left.)
As for the real case, I've done things to my phone that shouldn't be done to trash, and there's not a mark on it - or on the plastic cover for the screen. An Otterbox (I have the Defender, but the Commuter is just as strong) is worth the money.