Previous research has found that ethnic gaps in first-year university academic performances are substantial. We utilise unit record administrative data made available by the Strategy and Planning Department at AUT (Auckland University of Technology) to explain the possible origins of these ethnic gaps in academic achievement. These potential factors include socio-demographic characteristics, university admission types, and pre-university academic achievement. We examine the academic success of first-year students enrolled in Bachelors’ degree programmes at AUT between academic years 2012 and 2015. We furthermore decompose the ethnic gaps in academic outcomes observed between Maori, Pasifika, Asian and European students into their explainable and unexplainable components. Our findings show that European students were more likely to ‘succeed’ than the three minority ethnic groups in terms of both paper completion outcomes and letter grades. The NCEA rank score was the single most powerful variable in explaining these ethnic disparities. Furthermore, identifying the contributions of different factors in explaining the ethnic gaps in first-year university outcomes provides important guidance for university administrators and policy makers in designing programmes and policies to reduce overall ethnic gaps for students when they first begin their university study.