 The island subspecies of tomtits show a striking variation in body size, being considerably larger than their mainland relatives, a tendency known as the Foster's rule or the island effect.Birds from the main islands weigh around 11g, compared with birds from Snares Island, which weigh in at 20 g.

The tomtit is mostly an insectivore, feeding on small invertebrates, such as beetles, caterpillars, spiders, moths, weta, earthworms, and flies.Fruit is taken during the winter and autumn.