Not an easy question to ask with the rifles that are available here in Aus. I would say that you need to look at it from the point of view of what materials are available here and how best to use them.
I would start with the cases and projectiles that are easily available and get a rifle built to suit.
With projectiles in something bigger than .338 calibre you might have to go with a monolithic soild such at the "Predator Projectiles" in order to keep a high B.C. They make a range of bullets and they are not expensive. You may need to sort them to get a batch of accurate ones but the ones I tried in .338 calibre seem to shoot OK. Maybe .408" or .458" calibers would suit you?
Then you would need an easily available case to build it on. How about the .500 Jeffery case?
The .500 Jeffery case is available from Norma and also Bertram right here in Victoria. The .500 Jeffery can shoot a 570 grain bullet at 2200 fps so you should be able to neck it down to a smaller caliber and fire a 350 to 450 grain bullet at about 2500 fps to 2800fps? This would make an animal weak at the knees at quite some distance.
Then all you need is for some Aussie "Blacksmith
" to provide a suitable action and reloading dies and put it all together for you. But I have to be there when you fire it for the first time.
I would be interested to hear what other ideas some of the more experienced big banger shooters have to say on the subject.