Introduction Schools play a significant role in influencing childrens physical activity and sedentary behaviours and are a key recommended setting for physical activity initiative globally. To achieve population-wide health benefits, they must be guideline-informed and widely adopted. However, evidence on their implementation in Australian primary schools is limited. Objective To assess the reported implementation of recommended physical activity initiatives in Australian primary schools and explore their associations with school characteristics. Methods A cross-sectional study surveyed a nationally representative sample of Australian primary school principals (August 2022-October 2023). Principals reported on 33 physical activity initiatives categorised by delivery context: in the classroom; outside the classroom and during break times; outside of school outside or involving families; or other. Prevalence estimates were weighted to the Australian school population, and logistic regression models assessed associations with school characteristics. Results A total of 669 schools participated, with 360 principals completing the survey. Reported implementation rates varied from 4% to 98%. The most frequently implemented initiative in classrooms was Physical activity units of work in PDHPE/HPE curriculum across all year groups (98%); outside the classroom/break times was School infrastructure that supports physical activity during breaks everyday (e.g. play areas) (96%); and outside of school/involving families was School provides end of trip facilities to encourage active school travel (75%). Implementation of nine initiatives was associated with school size (n=6), geographic location (n=4), or socio-economic status (n=1). Conclusion This first national study provides critical information on current implementation levels individual physical activity initiatives in Australian primary schools and highlights where policy and practice investment in implementation support is required.