A social identity framework was employed to understand why people support the exclusionary treatment of refugee claimants ('asylum seekers') in Australia. Over and above individual difference effects of social dominance orientation and individuals' instrumental threat perceptions, insecure intergroup relations between citizens and asylum seekers were proposed to motivate exclusionary attitudes and behaviour. In addition, perceived procedural and distributive fairness were proposed to mediate the effects of social identity predictors on intergroup competitiveness, serving to legitimise citizens' exclusionary behaviours. Support for these propositions was obtained in a longitudinal study of Australians' social attitudes and behaviour. Small and inconsistent individual-level effects were noted. In contrast, after controlling for these variables, hostile Australian norms, perceived legitimacy of citizen status, and threatening socio-structural relations were strongly and consistently linked to intentions to support the harsh treatment of asylum seekers, and exclusionary attitudes and action at Time 2. Moreover, perceived procedural and distributive justice significantly mediated these relationships. The roles of fairness and intergroup socio-structural perceptions in social attitudes and actions are discussed.