The southernmost population is on the Chatham Island group, Mangere Island I think is the southernmost with nikau. If you browse through online real estate listings for Chatham Island you often find these palms in pictures. ON the mainland the population at Akaroa near Christchurch is the southernmost on the East Coast, while on the West Coast the populations end around Greymouth (I think). Occaisonally I hear stories about populations further south, but never confirmed.
These palms are not likely to be the most cold tolerant Rhopalostylis at all, as the southernmost areas are either totally or nearly frost-free coastal sites. The Chatham provenance should certainly be the best bet for very cool summer near frost-free climates (coolest parts of San Francisco etc), and is also a very attractive form.
If frost tolerance is wanted I suspect some of the more inland and higher altitude sites on the North Island would be better. Some of these populations would see -5C most years. There is a population easily seen form the road in the Waioeka Gorge which are in a spot cold enough that sometimes the frost stays on the ground all day in winter. Nice formed palms as well. There are likely better candidates than this elsewhere.
The one thing that will not be found is a population that experiences both cold and heat. NZs climate is mild by international standards, and most of the nikau populations in habitat (usually rainforest remnant) would see extreme extreme annual temperatures inside the range from about 35-90F. They do handle temperatures outside this range in cutivation of course.
