In 2018, the Counting Ourselves: Aotearoa/New Zealand Trans and Non-Binary Health Survey recruited a total of 1,178 transgender people aged 14 or older (Mage = 29.5) who lived in Aotearoa/New Zealand. This comprehensive survey included mental health measures from Aotearoa/New Zealand population-based surveys, as well as questions specific to experiences of being a transgender person, which were either adopted from overseas transgender surveys or developed in consultation with the project’s community advisory group. The study recruited participants using various recruitment techniques, such as advertising on social media (e.g., Facebook), making connections with transgender community organisations, and reaching out to the network of academic researchers and health professionals working in the field of transgender health. This thesis comprises two review studies and three empirical studies that report findings from the Counting Ourselves survey. Informed by the health equity perspective of LGBTQ-affirmative psychology, these studies fill in the literature gap related to transgender people in Aotearoa/New Zealand by (1) critically reviewing existing literature on Gender minority stress theory, and putting forward a framework that aligns with the understanding of cisgenderism as a marginalising prejudice for transgender people; (2) drawing on existing transgender research in Aotearoa/New Zealand to provide an overview of the social determinants of mental health for transgender people in Aotearoa/New Zealand; (3) examining the extent of mental health inequities affecting transgender people relative to the Aotearoa/New Zealand general population across all age groups; (4) exploring the predictive power of transgender-specific enacted stigma and protective factors on mental health outcomes of transgender people; and (5) using an inductive thematic analysis to analyse qualitative comments from an open-ended question to understand the nuances of mental health indicators affecting transgender people. Findings from empirical studies noted large inequities in mental health between transgender people and the Aotearoa/New Zealand general population, and the differences were especially prominent for those in younger age groups. Enacted stigma or overt experiences of gender minority stress (e.g., discrimination, harassment, and violence experienced for being transgender) were associated with elevated rates of mental health problems, while protective factors such as support and connection from friends, family members, neighbourhood, and transgender communities were associated with better mental health. Besides gender minority stress experiences, qualitative analysis revealed other mental health determinants that were important for transgender people, such as the ability to affirm their gender, equitable access to gender-affirming care and mental healthcare services, and support from families and the wider community. Overall, this thesis addresses important literature gaps by providing insight into the associations of enacted stigma and protective factors with mental health inequities among transgender people in Aotearoa/New Zealand. The reported findings have crucial public and healthcare implications, which include the need to promote anti-discriminatory practices against transgender people and trans-cultural competency in healthcare settings. Furthermore, this thesis evidenced a need to move beyond pathologising approach that views transgender people as “deficit” when understanding their mental health experiences. Instead, this thesis highlights the importance of examining enacted stigma related to cisgenderism and social determinants of mental health for transgender people.