One of the aims of Maori-medium education is to address Maori language loss. One of the challenges facing Maori-medium educators is to identify configurations that acknowledge the substantive importance of English language instruction without detracting from the priority that must be given to the regeneration of the Maori language. Issues relating to Maori/English bilingualism and assessment development in the New Zealand context are introduced and discussed in the light of local and international literature on language acquisition and other related fields. This paper also presents and compares the results of testing from 1995 and 2002-2003 using a reconstructed standardised assessment in literacy for Year 2 students in 80-100% immersion in Maori as a measure of literacy and Maori language acquisition.