Intervention at scale with the aim of improving student participation, engagement and outcomes in mathematics education is a challenge for educational policy makers and reformers. This article argues that an iterative annual cycle of policy formulation, implementation and evaluation enabled ongoing adjustments to the strategic focus, the professional development model and the system infrastructure as the New Zealand Numeracy Development Project was taken to scale. The analysis draws on the project's evaluation data over a 6-year period to demonstrate how adjustments were made over time to the pedagogical tools and to the professional development processes. The ongoing development of knowledge supported the management of strategic risks in taking the project to scale: the ongoing appropriation of adequate levels of resourcing to support the school-based professional development model and the availability of system-wide expertise for effective implementation. The analysis suggests that conceptualising implementation as an interdependent and interrelated component of an iterative policy process and as an opportunity for knowledge building ensured a continuing focus on student outcomes. The dynamic approach to the policy process appeared central to building this intervention's effectiveness and feasibility at scale.