Hi Jules - yes it is sparingly naturalised in New Zealand (first recorded here in 1995). I have seen it wild in the Waikato mostly, It is not particularly invasive though - it spreads mostly by suckering and occasionally by seed (it rarely sets fruit). Its an attractive plant that prefers a cool climate and good soil - you indicate you are from the far north - I have not seen it thriving in gardens north of Warkworth. While I understand your interest Astrid is correct, it may be worth looking at some indigenous alternatives but if you pick Kerria, then at least, as far as I know its not likely to be troublesome plant in your garden or outside it.