Parents of children on the autism spectrum and with rare genetic neurodevelopmental disorders (RGND) often experience higher stress, mental health difficulties, and social isolation than parents of children without developmental disabilities (DD). This study sought to expand on the current literature within the cultural context of Aotearoa New Zealand, by examining the overall mental health and wellbeing of parents with and without such children. We also examined the impact of social and economic disadvantages on parents and how these affect their stress, mental health, and social support. The study used ecological momentary assessment (EMA) methods, delivered via the smartphone application ExpiWell and online through Qualtrics, to assess parents' mood, health behaviours, and daily activities bi-weekly for seven weeks. We recruited three groups, parents of children on the autism spectrum (n = 29), parents of children with an RGND (n = 19), and parents of children with no DD (n = 29). Participants did not differ on social and economic factors. However, we observed relationships between parents' experience of social and economic disadvantage with increased feelings of stress, mental health difficulties, and lower social support. Our study also found that parents of children on the autism spectrum or with an RGND report higher feelings of stress and lower social support than parents of children with no known DD. Despite the differences in stress, mental health difficulties, and social support observed between the three groups, children's problem behaviours were the primary predictor of whether parents had a child on the autism spectrum, with an RGND, or with no known DD. Overall, our results support previous research findings indicating that parents of children on the autism spectrum or with an RGND experience higher levels of stress and lower social support than parents of children with no known DD.