A 7-month mathematics proficiency program was conducted in a primary Australian Indigenous community school. This paper focuses on outlining the specific methodologies employed to explore how students' mathematical proficiency changed throughout the implementation of the program in Years 2 to 4 (~ 7 to 9 years old). A mixed methods research design was utilised, and findings will be presented to evidence how the combination of standardised tests, diagnostic tests, and Newman interviews were useful in capturing and making visible young Indigenous student's mathematical proficiency. Whilst standardised tests provided a useful and comparable measure of student achievement, diagnostic tests and Newman interviews gave space for Indigenous student voice and demonstrated their strengths and areas for improvement in relation to their conceptual understanding, procedural fluency, and strategic competence. From these findings, recommendations concerning the adjustment of data collection procedures for young students in this setting are presented. The findings question the accuracy of standardised tests in revealing young students' proficiency, and this has implications for the extent to which standardised tests are relied upon to inform educational reform particularly for Indigenous students. Striving for equitable educational outcomes is an important endeavour in Australia, and such undertakings must be driven by meaningful and accurate evidence of students' proficiency in mathematics.