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Objective: This study aimed to describe and compare the proportion of patients classified as an emergency department (ED) mental health presentation under different definitions, including the Australian Institute of Health and Welfare (AIHW) definition. Methods: This retrospective cohort study enrolled all patients that presented to the EDs of a multi-centre Victorian health service between 1 January 2020 and 30 June 2023. Varying definitions of a mental health presentation were applied to each ED attendance, applying the current AIHW definition (using selected diagnosis codes), broader diagnosis-based coding, the presenting complaint recorded at triage and whether the patient was seen by or referred to the emergency psychiatric service (EPS). The proportion of all ED presentations meeting each definition and any overlap between definitions were calculated. The agreement between each definition and the AIHW definition was evaluated using Kappa's coefficient. Results: There were 813,078 presentations to ED of which 34,248 (4.2%) met the AIHW definition for a mental health presentation. Throughout the study, 45,376 (5.6%) patients were seen and/or referred to EPS, and 36,160 (4.4%) patients were allocated a mental health presenting complaint by triage staff. There was moderate interrater agreement between these definitions, with a kappa statistic (95% confidence interval) between the AIHW definition and a mental health presenting complaint recorded at triage of 0.58 (0.58–0.59) and between the AIHW definition and review by EPS of 0.58 (0.57–0.58). Conclusions: The AIHW definition is a conservative measure of ED mental health presentations and may underestimate emergency psychiatry workload in Australian EDs. What is known about this topic? It is unknown whether the Australian Institute of Health and Welfare (AIHW) definition of an emergency department (ED) mental health presentation definition accurately reflects ED mental health workload. What does this paper add? The AIHW definition appears to underestimate the clinical workload related to ED mental health presentations. There is moderate agreement between the AIHW definition and mental health presentations classified according to complaint recorded at triage, or whether the patient is reviewed by psychiatry services. What are the implications for practitioners? Accurate measurement of emergency psychiatry workload should not rely on the AIHW definition alone. [ABSTRACT FROM AUTHOR]