Education research journals regularly report on small-scale studies that have been successful in changing mathematics teachers' classroom practices. But it is rare to find professional development programs for teachers of mathematics that have been scaled up and sustained over time. This article reports on a research-informed, large-scale professional development project initiated by a state education system in Australia that now involves a large number of schools and teachers. The project developed a cluster model for bringing together primary and secondary school teachers to build their curriculum knowledge, confidence, and enthusiasm for teaching mathematics. The study aimed to identify factors that contribute to the sustainability and scaling up of this initiative. Drawing mainly on interviews with 61 participants -- teachers, school curriculum leaders, principals, and regional education leaders -- the study identified factors related to professional development content, collective action, and leadership at the school and regional levels. The findings contribute to the literature on mathematics education reform and identify implications for supporting teachers and schools in this enterprise.