The European discovery of the Chatham Islands in 1791 resulted in significant consequences for its indigenous Moriori people. The colonial Australian influence on the Chathams has received little scholarly attention. This article argues that the young colonies of New South Wales and Van Diemen’s Land led the exploitation of the archipelago before its annexation to New Zealand in 1842. The Chathams became a secretive outpost of the colonial economy, especially the sealing trade. Colonial careering transformed the islands: environmental destruction accompanied economic exploitation, with deleterious results for the Moriori. When two Māori iwi (tribes) from New Zealand’s North Island invaded in 1835, Moriori struggled to respond as a consequence of the colonial encounter. Mobility and technology gained from the Australian colonies enabled and influenced the invasion itself, and derogatory colonial stereotypes about Aboriginal peoples informed the genocide that ensued. Hence this article writes the Chathams into Australian history and Australia into Chathams history, showing that discussions of the early colonial economy, environment, and genocide must consider the wider South Pacific context in conjunction with events internal to the colonies.