Durable architecture is a hallmark of Polynesian chiefdoms, associated with centralised control of residential and agricultural land. Previous work in West Polynesia has indicated a relatively late date for the onset of such construction activity—after AD 1000—suggesting that political development was influenced by events such as post-colonisation migration. The authors report new dating evidence from the excavation of a large earth mound on the island of Tongatapu. Its construction 1500 years ago indicates that, in contrast to previous findings, well-developed chiefdoms and field monuments probably dominated the landscapes of West Polynesia substantially prior to the colonisation of more easterly island nations.