The successful protection of sites of high conservation value from the unintentional introduction of species necessitates an understanding of the drivers and impediments to action, and the manner in which research, policy development and praxis interface to constitute an appropriate level of protection - in theory and in practice. Factors bearing on how the quarantine protection of such sites is constituted are usefully explored in relation to islands - on account of the heightened planning considerations associated with their geographies and destination appeal, and their relatively small size, notionally defendable edges and oft-cited vulnerability and importance as refuges for rare and endangered species. Eight Australian Islands designated IUCN Category 1a Protected Areas (Strict Nature Reserves) were selected as case studies using a mixed, purposeful sampling strategy that deferred to each case's likely intrinsic merit in illustrating a diversity of protection issues, and in providing balance and variety of information. Insights into biosecurity praxis were elicited by conducting semi-structured interviews with policy makers, land managers and the proponents of island activities; reviewing the grey literature of stakeholder agencies; and observing quarantine management practices. Factors found to impact upon praxis included perspectives held about the activities providing for introductions and the extent to which these pathways can and need to be managed, how non-indigenous species issues are articulated and scoped, the sometimes default and problematic reliance placed on quarantine arrangements constituted at the national level, the degree to which land management agencies and land users are compelled to act and perspectives on the efficacy of past protection efforts.