Provided you sell them that is. If you hold them, why would you declare them? I mean, with that logic you would still have to declare them while they're here. Also, if you were to sell them here, you would have to declare them anyway.
So assuming you were going to sell them and you made a capital gain, you would have to declare those to the ATO anyway, which will mean the government will know you have an interest in PMs. So whether the government knows you have PMs by declaring it in your tax return or they know because you made an electronic transfer to Guardian and it left a paper trail, they end up knowing in the end (although it might not be in full picture).
Also, who says that people only store PMs in guardian? they could possibly store stuff that are not PMs (secret documents etc). So if you made an electronic transfer to Guardian and they report it, it just says to them that you have a storage account with Guardian. Having a storage account with Guardian shouldn't mean that you own PMs.
So once again, how big of a problem is this in its current form where changes will be made in April 2020?