Sphenodon guntheri (Brother Islands tuatara) was not recognised until 1989. Sphenodon guntheri (Brothers Island tuatara) are characteristically smaller and have longer reproductive cycles than Sphenodon punctatu.
Sphenodon guntheri inhabits the North Brother Island and Matiu/Somes Islands in Cook Straight, New Zealand (41°06? S, 174°26? E). Only a few hundred individuals remain on North Brother Island.

Sphenodon guntheri are one of the few reptiles with the ability to thrive in cooler conditions. They are nocturnal and physically capable of withstanding temperatures as low as 9°C, with humidity in the range of 70 to 80%. High humidity and low temperatures allow tuataras to maintain healthy shed cycles and live longer life spans, due to their effects on heart and metabolic rates. During the day, most individuals inhabit burrows along cliff faces. Their burrows can measure about 5 meters in length and 30 centimetres in depth, and are sometimes taken over from previous inhabitants. Burrows are typically found in open areas featuring low coastal vegetation, and usually offer both shade and sunlight to aid in heat regulation. Certain cliffs and other areas of the island that provide different types of terrain are often inhabited by birds or other animals that can compete with the tuatara for territory. (Cree, 1994; Lutz, 2005; Mitchell, et al., 2008; Nelson, et al., 2002a; Ramstad, et al., 2007)