Establishing the mathematical content knowledge of pre-service teachers is a requirement in New Zealand. However, this knowledge is not clearly defined and is a challenge for initial teacher education providers to assess. We develop the concept of foundation content knowledge and suggest how it can be assessed using two forms of diagnostic assessment. We examined an established conventional written test and an adaptive online adult numeracy assessment. The usefulness of each assessment tool was analysed in terms of how it revealed the foundation content knowledge of pre-service teachers. The two assessment tools were administered to different cohorts of pre-service teachers. In both cases, less than one-half of each cohort demonstrated foundation content knowledge at the beginning of their teacher education programmes. Both assessment tools provide information about the learning needs of each pre-service teacher. Crucially, the adaptive assessment tool gave immediate feedback directly to pre-service teachers. Furthermore, the adaptive assessment tool has the potential to provide consistent diagnostic information from year to year and between initial teacher education providers.