The demographic shape of New Zealand is changing as birth rates decline and life expectancy steadily increases. The effect of this is to greatly increase the proportion of the population in the older age groups. We report on a static microsimulation model of the primary care system in New Zealand used to test propositions about its functioning and development under different scenarios of demographic ageing. There were three central components: demographic ageing and associated morbidity, family and community capacity to cope, and practitioner and system response. Data from three different health-related surveys were combined to create a synthetic representation of the process and context of primary health care. We describe the construction of the model and present results quantifying a range of scenario options for policy development. We conclude with assessing the potential benefits of our modelling approach and our specific model.