<p>University-school partnerships are increasingly recognised for their potential to bridge theory-practice divides and foster sustainable professional learning. This case study examines how a Design-Build-Test-Learn (DBTL) professional learning model, implemented in an Australian independent school partnership, supported the development of teacher collective efficacy. Data from surveys and semi-structured interviews revealed partnership mechanisms explicitly enhancing Bandura’s four sources of collective efficacy: mastery experiences, vicarious learning, social persuasion, and positive affective states. Specifically, iterative collaborative cycles and targeted peer interactions facilitated mastery and vicarious experiences, while strategically structured dialogues and autonomy-supportive environments strengthened social persuasion and teachers’ positive affect. Findings further highlight distinct but complementary roles: universities as cultivators of professional learning cultures, and schools as contextual experts driving practice-based innovations. This study addresses critical gaps in existing literature by clarifying partnership roles and explicating mechanisms supporting under-explored sources of collective efficacy, thereby providing insights into fostering sustainable professional learning cultures through intentional partnership designs.</p>