Singing and territorial behaviour of North Island tomtits (Petroica macrocephala toitoi) were used to monitor population size over a 3-year period at Atuanui, Mount Auckland Scenic Reserve, North Auckland. Male tomtits were observed singing year-round with singing peaking in the period from Nov to Jan. The general territorial behaviour of Atuanui tomtits was similar to that reported for other North Island populations, with territorial males resident in all months and most territories occupied in successive years. Density of territories was stable over the 3-year period but vacancies in suitable habitat suggest the population is not at carrying capacity.