megasupermagnum
45 Cal.
Not outlawed at all, just need to be registered.
I'm not trying to insult you, but quit blowing smoke. Australia is one of the strictest countries in the world. Top 5 for sure, right behind China and North Korea, which are about the only countries which have outright banned no exceptions firearms to a normal person.
Now onto something that actually matters to this thread. Due to your circumstances, I'm going to suggest you do not get a 58 caliber. Normally I'd say just buy whatever makes you happy, but 58 caliber isn't much of a fun caliber if you ask me. It is a hunting workhorse. If you were just looking to have fun, go smaller. If you are hoping 58 offers something 54 doesn't for hunting, you are going to be disappointed. 54 is perfectly suited for Elk and Red deer, even on steeper angle shots. If you want more, then go bigger. 62 caliber is a worthwhile jump from 54, and I wouldn't be scared to go bigger yet if you could find the right rifle.
In my own opinion I don't see much point in making small changes to caliber. Like most, I own both 50 and 54 caliber. I think it is a pain. The difference is negligible, and you always have the wrong jag, or balls, or whatever. I still own a 50 caliber, but for the most part I standardized on 54 caliber, and don't regret it one bit. I shoot my various rifles and handguns just as much as if they were all different calibers, probably more so. If all you had was a 50 caliber, I'd say a 58 might be worth it, but 54 to 58 is nothing.