The islands of Polynesia are often seen as â€œnatural laboratoriesâ€ where population size and growth are core components of models which examine the development of Oceanic societies. As in many parts of the world, the population of Pacific Islands suffered severe declines after contact when Europeans introduced new pathogens. These rapid population declines and a historical record weighted heavily to later missionary accounts complicate estimates of population size and distribution. Here, we use lidar derived settlement data to estimate the population of the island of Tongatapu, the seat of power of the Tuâ€˜i Tonga polity between the 13th and 19th centuries. We use a Bayesian approach to adapt the house count method and estimate population based on information from: the number and spatial distribution of households from lidar survey, archaeological transect data to determine structure function, and ethnographic records of household composition. Results indicate the population of Tongatapu was probably between 50,000 and 60,000 people, suggesting a total population living in the archipelago of between 100,000 to 120,000 people. Population estimates were compared to population retrodictions calculated with previously unanalysed historical sources to show the Tongan archipelago suffered a population decline at all island groups of approximately 70% to 86%. These results highlight the contribution lidar derived settlement data to our understanding of demography and social development.