Prospective teachers' mathematics efficacy beliefs affect school placement experiences and influence later teacher behaviour and subsequent student outcomes. These efficacy beliefs are open to change during initial teacher education and become more resistant to change once the teacher enters the workforce. Therefore, it is important to measure mathematics teacher efficacy early in initial teacher education and identify low efficacy beliefs. This study measured the mathematics teaching efficacy beliefs of 402 prospective primary teachers. Arising from Rasch analysis of the efficacy scores of these teachers, 22 participants who displayed a range of mathematics teaching efficacy beliefs were interviewed to gain insights into factors affecting these beliefs. Participants communicated greater confidence in tasks requiring procedural (tell-items) than conceptual knowledge (explain-items). Both low and high efficacy participants reported traditional experiences of mathematics teaching and learning as students. However, high efficacy participants reported mastery of mathematics within these traditional contexts, whereas their low efficacy peers did not report mastery. Furthermore, all participants reported how memories of their school mathematics experiences informed their level of efficacy when thinking about teaching mathematics, thus revealing the powerful influence of mastery experiences.