This paper provides an introduction to the phonology of Banyaduq, a Land Dayak language spoken in western Borneo, which, previous to this study, has never been described in the literature. We focus on the prestopped nasals of Banyaduq, which are of particular interest because, in some but not all dialects, the distribution of prestopped nasals is not predictable. Thus, in those dialects, prestopped nasals are phonemic. This situation contrasts with the description of prestopped nasals in other languages in the region, in which they are described as predictable variants of simple nasal stops. We argue that there is a direct connection between the phonemic status of these prestopped nasals and a diachronic process, noted by previous researchers in a variety of Austronesian languages, in which prestopped nasals become oral stops. We propose that phonemic prestopped nasals are an essential intermediate step on the diachronic path from allophonic prestopping to oral stops. We argue that the change to oral stops is natural for phonemic prestopped nasals, but not for prestopped nasals that are simply allophones of clear nasals. Our data are taken from three varieties of Banyaduq, with most of our data coming from Sangke Banyaduq, the native language of one of the authors.