Over the past twenty years the incidence of both childhood acute lymphoblastic leukaemia (ALL) and type 1 diabetes have risen in many developed countries, including New Zealand. Although the explanations for this increase and the precise aetiology of both diseases remain unclear, environmental factors are thought to be important. One factor receiving increasing attention is the role of infections introduced through population mixing. However, previous studies on this topic show mixed results and population mixing itself tends to be under-theorised. Furthermore, this issue has not been adequately assessed in New Zealand, a country characterised by high levels of population mobility. In this research, a variety of population mixing measures for small areas in New Zealand were developed. National data on ALL registrations were obtained from the New Zealand Cancer Registry, and regional type 1 diabetes data were obtained from the Canterbury Diabetes Register for the Canterbury Region of the South Island. The analyses were undertaken in three stages. First, standardised incidence ratios of each disease were examined at different geographical and temporal scales, between areas of differing socioeconomic status, and in urban and rural New Zealand. Second, cluster analysis was employed to test for spatial-temporal clustering of the two diseases. Finally, multivariate regression analyses were utilised to investigate the association between each disease and the various measures of population mixing at the area-level. The results reveal similarities in the geographical epidemiology of childhood ALL and type 1 diabetes in New Zealand. The majority of the findings were suggestive of an infectious aetiology for both diseases. In addition, higher incidence of both diseases was observed in areas which increased the most in population mixing over short time periods (6/7 years). Furthermore, raised type 1 diabetes incidence was also associated with high population mixing in early life.