Thats a real desaster for the Perth Mint. Even if its just a tiny deviation from the claimed purity, this is all about reputation and trust and all that. So it could even be a false claim by someone who wants to damage their business.
So I dont believe the actually diluted the purity. Thats not even neccessary if you purify gold. That fourth 9 is extremely difficult to achieve, and there is always a bit of silver left. Thats why the cant get to a 100% purity. But if they messed up and sold gold that has not the purity which is stated, it will be a pretty bad blow for them, and it will get expensive to rectify the mistake.
However, I would be surprised if something similar has happened before with other gold producers. I mean, who really checks the purity, if you buy stuff from the US mint, or the Bank of England, or some other big name? and such small discrepancies you cannot even detect by the simple standard tests.
Btw., I dont think this is a probelm with silver. Silver is not expensive enough to make it worth fiddling with it. Of course there are some con men who try to sell you silver plated fake maple leafs on ebay and so on. But the silver producers wont try such stunts, I guess.