BACKGROUND: Use of evidence to inform health policy and planning decisions is important to ensure effective, efficient and equitable interventions, yet there has been little examination of decentralized, regional health bodies. This study aimed to examine the extent, purposes and sources of evidence used by Australian Primary Health Networks (PHNs) to inform regional primary health care planning, and explore conceptions of, and attitudes towards evidence. METHODS: We conducted document analysis of all 31 PHNs' Needs Assessments, Activity Work Plans and Annual Reports, and conducted 29 interviews with key stakeholders from a sample of five PHNs. RESULTS: We found that PHNs used evidence to identify health needs to a far greater extent than to inform the planning and development of primary health care interventions. The evidence used largely consisted of quantitative data from government sources. There was very little use of evidence from research or evaluation documents. Evidence from community and other stakeholder consultation was useful for complementing quantitative data with localized knowledge but was of questionable rigour. Conceptions of evidence were generally broad. Interviewees tended to favour quantitative evidence, and the evidence that aligned with their professional background. CONCLUSIONS: We recommend PHNs improve the use of intervention evidence and stakeholder engagement to inform effective, efficient and equitable local PHC initiatives. Developing more robust, transparent and standardized internal processes for evidence-informed program planning and evaluation, as well as maintaining and strengthening community and stakeholder participation in the planning process, will improve the robustness and effectiveness of planning.