Well even though Max is an Australian character, you could kind of hear, even in the Mel Gibson movies, a tiny bit of American accent slipping in at times. I mean, in real life that's because Mel wasn't born in Australia and held some holdover from his childhood accent, but in-universe you could chalk it up to American influence on Australian culture. And the thng about the Aussie accent is that it varies depending on which part of the country you're from, and sometimes as weird as it is, some people have the slightest American inflection if they've grown up in the bigger cities.
But ignoring all that, it's a post-apocalyptic future. Maybe there was a larger immigration of American refugees in Australia in the Mad Max-verse.
It makes it seem like he doesn't really know what to make of this whole thing. Well, having lost his family and wandering a desert wasteland alone, might make him a little disconnected with other human beings. I didn't find it awkward.
