Born in Northern Ireland to army parents, his family returned to the South Island of New Zealand in 1954. [...] worked with the New Zealand Players and other theater groups. He also was a film director, editor and scriptwriter for the New Zealand National Film Unit for 6 years. His first feature film was Sleeping Dogs (1977). He then moved to Australia and [...]So he moved -- or rather, "was moved" -- "home" to New Zealand in '54, at ~seven years of age, and moved from there to Australia some time in the '77 - '79 timeframe, when he would have been around thirty. (Shortly after that, he moved to Britain, and then "moved back to Australia in the late 1980s").
I'd say that makes him pretty definitely a New Zealander, since that is where he spent his formative years; and secondarily, if anything, apparently an Aussie. (Then again, perhaps Aussies and Kiwis alike are all "secondarily" Brits anyway?)
Commenting on Neill as an actor, let me add: IMDB quotes his 'Biography from Leonard Maltin's Movie Encyclopedia' - "If Neill is still not quite "star material," there is no question of his status among the best, and most versatile, actors working today." Huh?!? And this doesn't tell them that there's something wrong with how they determine what is 'star material'? Aren't these 'stars' supposed to be 'star' *actors* in the frigging first place?!? 'Far as I'm concerned, Sam Neill *is* a bloody 'star' -- much more so, than most vacuous pretty-face Hollyweird creatures!
One of his earliest performances that I saw: [link|http://us.imdb.com/Title?0084157|Ivanhoe (1982)], a "breakthrough" for me personally in that it may have been the first fiml I saw -- at least, it's the first I definitely remember -- where "the villain" (Neill plays the ambivalent but mostly 'evil' Templar knight Brian de Bois-Guilbert) steals the show. (Together with James Mason, who -- perhaps not-so-coincidentally? -- was, according to IMDB, instrumental in bringing Neill to Britain and thus probably furthered his career a good deal.)