Background <p>Women who are pregnant or have recently given birth constitute a distinct life-stage-specific population with elevated risks for mental health problems and well understood risk and protective factors. Perinatal mental health remains a worldwide public health problem that requires local health service reform to improve access and meet need. The aim of this paper was to create a framework for a proposed model of postnatal mental healthcare and understand its application in existing health services in a rural area in Australia.</p> Methods <p>The Theory of Change (ToC) method was used in a three-step procedure: 1) document reviews 2) stakeholder consultations and 3) ToC workshops. A proposed model of postnatal mental healthcare was developed and its application in a local setting was investigated. The model was documented using a ToC framework.</p> Results <p>Drawing on evidence from policy documents, clinical practice guidelines, existing health systems, and practice expertise, a ToC for a proposed ‘stepped model of postnatal mental healthcare’ was developed. Stepped care is a complex intervention intended to deliver tailored care pathways according to severity of individual need. It links four interrelated components: primary prevention, secondary prevention, early intervention and treatment. In the ToC, evidence-based Rationales support each Pathway, and Assumptions describe external conditions that must be satisfied for successful implementation of each Pathway and the overall model.</p> Conclusions <p>The proposed stepped-care model is a valuable investigation of an integrated model of postnatal mental health service reform in a local service environment. The Assumptions generate testable hypotheses for future research to understand the conditions for successful implementation in this or other settings.</p>