Islands create unique environments that provide niche expansion opportunities for invading species not previously available on continental areas. This thesis investigates variation in house mouse mandible shape and body size across the New Zealand archipelago, and aims to identify environmental variables that might influence change between island populations using geometric morphometric techniques. Mice were introduced to New Zealand in the early 19th century with European explorers and settlers. The Mus phylogroups currently known in New Zealand include Mus musculus domesticus, M. m. musculus, and M. m. castaneus. Each of these subspecies has a distinct distribution throughout the archipelago, with M. m. domesticus dominating the northern New Zealand ‘mainland’. Past studies have shown habitat composition and co-existence with rats can significantly influence the activity and behaviour of house mice. Additionally, environmental events such as seed masting are known to provide abundant food resources that inflate house mouse density. Consequently, different habitats may promote variation in dietary components. Here I examined variation between house mice inhabiting five different forest environments across the North and South Islands. Significant variation in mandible shape and body size was found between all five forest populations. Annual rainfall was the most significant covariate with mandible shape. General variation in body size somewhat followed rising latitude, supporting the general ecological trend known as Bergmann’s rule. Habitat type, ambient temperature, and presence of rats were also significant predictors of body size under regression. Ecological complexity and biodiversity are known to vary with island size. Islands with greater landmass are associated with increased habitat and species diversity which support heightened competition and predation. Small mammals colonising diminutive, remote islands often experience ecological release from these interactions, leading to the largest cases of gigantism currently known. This study investigated variation between six populations of house mice on small, often very remote islands in the New Zealand archipelago. Significant variation in body size and mandible shape was found between all six populations, but again, rainfall was the most significant covariate with mandible shape. The general pattern in body size between island populations followed Bergmann’s rule, and is also consistent with previous studies that observe an inverse relationship between island size and body size, peaking at 50° latitudes. Ambient temperature, rainfall, and genetics were also significant predictors of body size. Significant variation was also found between all New Zealand mouse populations compared with samples from Sydney. Australia. Sydney was an important shipping port during the settlement of New Zealand. Modern Sydney house mice are probably descendants of the original population from which northern New Zealand domesticus mice are derived. Sydney mandibles clustered with extreme examples of variation in New Zealand mouse mandible shapes on separate axes. Collectively, these studies describe significant variation between distinct house mouse populations of the New Zealand archipelago, and lay the foundations for further research in this area.