The orange-bellied parrot (Neophema chrysogaster) is a critically endangered small Australian parrot. It was the first species in Australia to have a single species Recovery Plan developed, and efforts to conserve the species have been active for over 35 years. Despite this, the wild population has declined to fewer than 20 individuals. Approximately 450 birds are maintained in a captive insurance population. This thesis investigated genetic diversity across both wild and captive populations of the OBP between 2010 and 2018. Relatively low genome-wide diversity was found across the species, as measured with 7768 SNP markers. Low functional diversity was also found at immune genes the Toll-like receptors, consistent with other studies of critically endangered birds. Although genetic diversity in the wild population decreased following removal of 21 fledglings in 2010/11 to supplement the captive population, annual releases of captive birds since 2013, and their successful breeding post-release, have improved wild diversity levels. Wild and captive populations were not found to be genetically distinct. Inbreeding depression was investigated by modelling correlations between genetic diversity and 1) differential responses to infectious disease agents, and 2) reproductive success. No evidence of inbreeding depression was found, but a relationship between younger age and greater reproductive success was identified. Finally, a preliminary phylogeny of the genus Neophema was produced using two mitochondrial markers, and was found to support some of the current structure within the genus, but was ultimately inconclusive as to placement of the OBP. This work has explored genetic diversity in the OBP to a greater extent than ever previously. It has helped inform management of the species and will act as a foundation for future studies.