The vastness of the Australian mainland dominates understandings of colonial space. Instead, this paper analyses carceral islands off the coast of Australia. It argues that these sites enabled settler-colonialism within Australia and enabled imperial interconnectivity across the sea. It shows that Cockatoo Island (New South Wales) and Rottnest/Wadjemup Island (Western Australia) played key roles in the colonization of Australia. First, they were sites of expulsion for both Indigenous Australians and European ‘bushrangers’ who threatened the settler colonial frontier through livestock theft. The frontier was created in part through the dislocation of people onto islands at to the maritime margins of the colony. Second, it argues that islands were integral to the imperial project as sites of global connectivity. Convicts on islands built maritime infrastructure that allowed the Australian colonies to connect to imperial networks of trade. By focussing on the relationship between island peripheries, the Australian colonies, and the wider empire, the multi-sited nature of colonial governance and economies becomes clear. Using archival material from the Colonial Office, local correspondence and prison records, it views the history of Australian convict system from the outside in: showing that peripheral sites holding marginalised people were central to settler-colonial and imperial projects.