Introduction Within primary health care policy, there is an increasing focus on enhancing involvement with secondary health care, social care services and communities. Yet, translating these expectations into tangible changes frequently encounters significant obstacles. As part of an investigation into the progress made in achieving primary health care reform in Aotearoa New Zealand, realist research was undertaken with those charged with responsibility for national and local policies. The specific analysis in this paper probes primary health care leaders’ assessments of progress towards more collaboration with other health and non-health agencies, and communities. Aim This study aimed to investigate how ideas for more integration and joinedup care have found their way into the practice of primary health care in Aotearoa New Zealand. Methods Applying a realist logic of inquiry, data from semi-structured interviews with primary health care leaders were analysed to identify key contextual characteristics and mechanisms. Explanations were developed of what influenced leaders to invest energy in joined-up and integrated care activities. Results Our findings highlight three explanatory mechanisms and their associated contexts: a willingness to share power, build trusting relationships and manage task complexity. These underpin leaders’ accounts of the success (or otherwise) of collaborative arrangements. Discussion Such insights have import in the context of the current health reforms for stakeholders charged with developing local approaches to the planning and delivery of health services.