The aim of this study is to understand how New Zealand SSCOs successfully advocate for decreasing the negative effects of the determinants of health, as they affect marginalised communities. Community organisations influence the social and political environment, and advocate for the most marginalised communities. However, advocacy in the community sector is not well understood, and this may disproportionately affect groups like the resource poor because community groups are one of their primary advocates. If advocacy is limited in the SSCO sector, marginalised groups may not ever be heard.In light of this gap, a case study is used to explore advocacy in the social service community organisation (SSCO) sector in New Zealand. Armstrong and Bernstein's New Social Movement Alternative (NSM-A) and selected concepts from Bourdieu's field theory are applied to a single case study design with one embedded unit of analysis. Results from the larger case study of the community sector in New Zealand was drawn from 42 key informants, and 33 of the 42 interviews combined with a documentary analysis informed the embedded unit of analysis, the Family Centre.Results indicate that the SSCO sector in New Zealand directly serves the needs of communities with complex needs. SSCOs in New Zealand use multiple strategies when they advocate and the nature of advocacy in the SSCO field, like all fields, is marked by power, which influences how they engage in advocacy. The most successful, or "dominant", SSCOs are those that have credibility, a stable resource-level and a mission that corresponds with the policy or public agenda. Managing four relationships—with government, other community groups, the community and media—is important for increasing the success of SSCO advocacy.Three primary characteristics of successful organisations are a key finding of this research, which are all linked to SSCOs managing relationships effectively. First, influential community groups are in "dominant" positions within their fields. Second, effective organisations seek to influence multiple institutions. Third, successful groups manage relationships in two ways: they use credibility in one field to gain credibility in another and they bring together individuals from different fields.There are implications in three areas: practical, policy, and future research. Practical implications include recommendations for groups to move into a dominant position within their primary field, gain membership in multiple fields, and manage relationships in the ways described above. Policy implications are twofold; first, there are suggestions for clearer guidelines on advocacy from the New Zealand Charities Commission and second, a holistic approach to the community-government relationship is recommended. Five recommendations for future research are made. First, a relational theory is suggested and theoretical developments from this research are discussed. Second, an understanding how different subfields advocate is recommended. Third, more knowledge on indigenous organisations is needed. Fourth, more knowledge on Pacific organisations is also needed. Fifth, more research should be undertaken to understand what happens to the community voice in the process of advocacy.Overall, it appears that gaining credibility and managing relationships effectively are key factors in predicting the success of SSCOs. These findings contribute to the limited amount of research on advocacy in community organisations and provide the foundations for developing a relational theoretical approach. It is hoped that these findings will increase practitioners, policy-makers, researchers, and public health advocates understanding about the social service community sector and the centrality of SSCO advocacy in improving the health of marginalised groups.