Purpose: Instructional coaches are pivotal to articulating the agenda of system-wide reform, yet their role remains largely unexamined. Their approach with educators is contextually situated within the schooling system in which they work to reflect the historical and sociocultural system influences. Given the downward trend in New Zealand's international test scores for mathematics, it is timely to review the role of instructional coaches. Design/methodology/approach: In this paper, the authors draw on qualitative data derived from interviews with experienced coaches to investigate how they brokered the vision and pedagogy of a system-level reform in mathematics. Using a sensemaking lens we specifically examined the collective stories they employed as explanatory tools. Findings: The analysis revealed that coaches drew on factors from school and classroom contexts of professional development practice and from collective beliefs about effective practice, alongside the project materials incorporated in the design of the project. System-level stories of reading reform influenced coaches' leadership of professional practice in implementing the New Zealand Numeracy Development Project, a progressively scaled-up professional learning and development initiative designed to improve teacher knowledge and pedagogy. Originality/value: This paper highlights the critical importance of coaches' knowledge and expertise, the complexity of the implementation process and the coherence of the infrastructure that supports them in instructional reform.