This paper considers the application of Yang’s (2000, 2002, 2009) model of phonological change and population dynamics to the case of competing mergers in the formation of New Zealand English, as described by Peter Trudgill and colleagues. Trudgill (1986 et seq) argues for the deterministic nature of change in the specific case of contactinduced change referred to as New Dialect Formation, such that given sufficient knowledge about the linguistic features represented among the speakers of the different contact-varieties, it is possible to predict with a high degree of certainty the features which will survive into the new dialect. Specifically, Trudgill argues that the features that are in a majority in the input mixture will survive at the expense of its competitors. He accounts for exceptions to this generalization, the focus of this paper, in terms of linguistic pressures such as markedness. The aim of the current paper is twofold. First, Yang’s model predicts exactly the proportion of merged speakers necessary for a given merger to be successful in a competing grammar situation. Thus, we use the case of phonological mergers in the formation of New Zealand English as a case study to test Yang’s model. Second, the model can help us better understand the complexity of the dynamics of New Dialect Formation. Here, we test the hypothesis that New Dialect Formation is in principle no different from other types of language change, in the sense that the acquisition-based mechanisms driving language change are the same across all learners. Specific to New Dialect Formation, we argue, is the unique demographic situation through which the variation is introduced that forms the child’s primary linguistic data, the basis for his or her first language.