Internationally, there is a gap in high-school completion rates for Indigenous and non-Indigenous students. In Australia, gap estimates are commonly based on lag indicators, precluding examination of underlying mechanisms. Using two longitudinal representative samples of Australian youth, we explored differences in high-school completion between Australian Indigenous and non-Indigenous rates, and whether the gap varies for students of similar academic ability. Using an intersectional approach, we show the Indigenous gap is significant, is mostly a function of differences in academic achievement, but varies by socioeconomic status (SES) and location. Specifically, high SES and living in urban settings are protective factors for non-Indigenous students, but not for Indigenous students. Conversely, rural and poor non-Indigenous students appeared to have dropout rates as large or even larger than similarly poor and rural Indigenous youth. Overall, the results suggest the need for a more nuanced perspective on 'Indigenous gaps' in educational attainment.