Despite the growing political and economic importance of iwi in New Zealand, little attention has been given to their changing demography. Using data from the New Zealand Census of Population and Dwellings, we analyse the growth trajectories of four iwi - Ngāi Tahu, Waikato, Ngāti Awa and Tūhoe - between 1991 and 2006. We find a great deal of variation in iwi growth between censuses and over the entire focal period. In most instances, iwi birth cohorts increased rather than decreased in size, and gains from “new” members were especially marked among children, the middle-aged and women. The findings confirm the importance of accounting for non-demographic factors when projecting and planning for future iwi population growth.