Limited empirical research has investigated the impact of interdisciplinary STEM approaches on students' learning with a particular dearth in the area of assessment. A lack of alignment between interdisciplinary approaches in STEM teaching and its assessment strategies is notable. In this study, we adopt an inquiry approach to the ways in which interdisciplinary learning of science and mathematics impacted on student learning through the use of a STEM challenge that involved a real-world task. Two secondary school teachers, one who teaches science and the other mathematics, participated in the study and jointly planned and taught a sequence of science and mathematics lessons using an interdisciplinary approach to their Year 8 class of 24 students over several weeks. Data of students' project reports, video of their project presentation, and teacher interviews were analysed to identify the intersection of disciplinary specific and generic skills in assessment practice of an interdisciplinary task. Our findings highlight the importance of having an assessment rubric with criteria that addressed both disciplinary specific skills as well as generic STEM skills.