Professional Experience placements provide invaluable opportunities for pre-service teachers to connect their expanding knowledge to teaching practice. When done well, these experiences are underpinned by purposeful and continuous guidance from experienced mentor teachers. Significantly, the participation and engagement of Australian mentor teachers in this process is voluntary in nature, meaning the system relies on teachers 'opting-in' to mentoring. This research examines mentor teacher participation within Secondary-level initial teacher education courses and highlights issues relating to overall mentor teacher (and host school) engagement. Analysis of placement data over a five-year period to 2021 illustrates a significant change in participation with a dwindling number of mentor teachers participating in placement activity. Other insights include a declining rate of school and mentor teacher participation and an over-reliance on a portion of the teaching workforce to sustain these preparatory experiences. These findings highlight structural and systematic gaps negatively impacting on the delivery and quality of initial teacher education, which in turn have broad implications for the current national workforce shortage in Secondary teaching.