This thesis investigates change in a number of phonological variables in New Zealand English (NZE) during a formative period of its development. The variables under analysis are the short front vowels /ɪ/, /ɛ/, /æ/, the front centring diphthongs /ɪə/ and /ɛə/, and the so-called 'broad A' vowel. The sample includes 30 NZE speakers born between the 1890s and the 1930s (the 'Intermediate period'). Acoustic analysis reveals that the short front vowel system develops into one with two front vowels and one central vowel over the intermediate period via a push chain shift. There is evidence for complex allophonisation in the speech of early intermediate speakers. I argue that duration plays an important role in resolving overlap between vowel distributions during this time. With regard to the front centring diphthongs there is approximation of the nuclei of the two vowels in F1/F2 space over the intermediate period as well as incipient merger in the speech of late intermediate speakers. Although the merger is mainly one of gradual approximation, it is argued that patterns of expansion of the vowel space available to both vowels are also found. The analysis carried out on the 'broad A' vowel reveals that whereas flat A was still present in the speech of the earlier speakers from the sample, broad A had become categorical toward the end of the intermediate period. It is shown that, by and large, the process involves discrete transfer of words across etymological categories. The final chapters discuss a number of theoretical implications. Processes such as the NZE front vowel shift suggest that a number of previously recognised concepts, such as 'tracks' and 'subsystems', may either have to be relaxed or abandoned altogether. It is argued that chain shifts of this type come about by rather simple mechanisms that have a strong resemblance to functional principles found in the evolution of organisms. A case for 'fitness' of variants of a given vowel will be made. Phonological optimisation, on the other hand, is not a driving force in this type of sound change.