Well, World Englishes is not my own field so I'm not quite sure what the theories are as to why... NZE and AusE are quite distinct varieties just like AmE and BrE, and of course NZE and AusE are more like each other than they are like any other variety - and if you compare NZE and AusE to BrE and AmE, they will be more like one variety in some ways and more like the other in other ways. Still, I'm pretty certain that I have been told that there is a somewhat greater tendency in NZE towards AmE spellings and vocabulary, but of course I cannot find any sources for it right now.