A need for the successful and sustained implementation of positive and proactive approaches to behaviour support has been widely acknowledged in Australia. School-wide positive behavioural interventions and supports (SWPBIS) is one approach that has been increasingly adopted across Australia to meet the social and behavioural needs of all students. However, successful and sustainable implementation of SWPBIS with fidelity has proven a challenging task. At present, limited exploration of SWPBIS implementation efforts have been undertaken in Australia. The current study addressed these gaps in two ways. First, the fidelity of Tier 1 SWPBIS implementation was assessed within 15 schools. Results indicated the majority of schools were implementing Tier-1 SWPBIS with fidelity. Second, a survey instrument was developed, validated, and used to assess 241 teachers' perceptions of variables that may help or hinder SWPBIS implementation across these same Victorian schools. Based on these data, a two-step hierarchical regression model was used to identify predictors of implementation fidelity. The first model found leadership, resourcing, data, teacher beliefs about behaviour, and pre-service preparation were predictive of implementation fidelity, while the second found school duration of implementation, along with resourcing and data, predicted fidelity. The implications of these findings and future research directions are discussed.