Teachers' numeracy capability is essential for student learning in the classroom and important across all subject areas, not only within mathematics. This study investigated the use of online diagnostic tests as a form of assessment for learning, to evaluate and support teacher education students (TES) in developing their numeracy skills. Data were collected using the "Test" feature through the Blackboard Learning Management system at two Australian universities. In this paper, we report on trends amongst TES who showed growth in their numeracy capability through the repeated use of the diagnostic test.