It’s all taxation. Don’t pay your taxes? They take your stuff, lock you in a cage, and/or shoot you. If your store your value in anything other than AUD and it appreciates, you owe income tax on it at some point. This is typically when you “realize the gain”. Although, I have heard (never confirmed) that some countries tax unrealized gains in some circumstances.

I think I read somewhere similar things about the possibility of taxing unrealized gains too.

The problem is that it’s completely unfair as it’s unrealized. You actually haven’t made any gains at all, so you’re taxing something that may have artificially gone up in value, but then just as easily may go back down in value.