Objective The delivery of public out-patient services is an essential part of complex healthcare systems, but the contribution of public out-patient services is often ill defined and poorly evaluated. The aim of this study was to identify and better understand those factors that may affect the performance of out-patient services to provide health service managers, clinicians and executives with a conceptual framework for future decision-making processes. Methods The present qualitative research involved five exploratory case studies. These case studies were conducted across two specialties at hospitals in the Metro North Hospital and Health Service in Queensland. Data were obtained from 38 interviews and 15 focus groups, and were analysed to identify common themes. Further analysis helped identify the most significant factors and build a conceptual framework for understanding the relationships between those factors and their effect on performance. Results Across both specialties there were 10 factors (scheduling, performance, service framework, categorisation or prioritisation of patients, internal and external stakeholders, resources, service demand, culture, system challenges and medical stakeholders) identified that may affect the performance of out-patient services. These factors were condensed into five core domains: culture, stakeholders, resources, demand and system reform. Conclusion Strategies to address the five core domains identified may provide a framework for sustainable improvement in the delivery of out-patient services. What is known about the topic? The provision of specialist out-patient services is an essential element of health service delivery. Access to specialist services in the public sector is challenging because of the escalating demand associated with an increasing and aging demographic. The factors that may affect the delivery of out-patient services need to be addressed for long-term sustainable improvement. What does this paper add? This paper provides a conceptual framework grounded in rigorous qualitative data analysis for understanding the internal and external factors that affect waiting times for specialist out-patient services. The results of this qualitative research indicate that there are five core domains that may influence waiting times in the public out-patient setting. When these domains are addressed at the strategic, tactical and operational levels, they have the potential to provide significant improvement in the delivery of out-patient services. What are the implications for practitioners? This paper guides the attention of relevant stakeholders towards the five core domains identified (culture, stakeholders, resources, demand and system reform) that influence the performance of waiting times at the operational, tactical and strategic levels within the public hospital setting.