The Repeat Offenders Database, which has been collated by the New South Wales Bureau of Crime Statistics and Research, offers a unique opportunity to analyse data quality issues for an important source of administrative data for Indigenous people. This paper provides several independent estimates of the population of Indigenous offenders by estimating the number of people with unknown Indigenous status who are likely to be identified as Indigenous in other circumstances. The main finding is that the Indigenous population of offenders are substantially undercounted in administrative data collections. The failure to account for this will understate the ‘gap’ between Indigenous and non-Indigenous outcomes. (Abstract.)