Māori language and culture immersion programmes have been established now in Aotearoa New Zealand for about 30 years, however there is still not a great deal of research on the proficiency of the children who attend those immersion programmes. The Tuhinga Māhorahora project has two goals. The first is to test ways of providing timely information to classroom teachers that they can feed back into their curriculum planning and classroom practice. The second is to build a corpus which can provide information of use to those producing curriculum resources in Māori. The research project is collecting and analysing written texts written in te reo Māori by young learners in Māori immersion settings. The focus is on the vocabulary the learners produce during free writing sessions. These are sessions in which the writers choose their topic and write independently of the teacher. The researchers have collected writing samples into a corpus of approximately 67,200 words to date. We report on our methodology in establishing the database and results and challenges to date.