A .45/70 can certainly kill a water buffalo, kill a cape buffalo, kill an elephant, or all sorts of stuff. With the right bullet, right load, and if shot in the right place. And the right circumstances.

There is no legal minimums in Australia, so whatever you can work out with your outfitter.

Lots of cape buffalo were shot with .303's, .30-06's, 8mm's too before legal minimums were introduced.

Most outfitters would recommend a .375 or something similar though as a preference.