OBJECTIVE: This study aimed to determine the factors associated with extended length of stay (LOS) for paediatric mental health (MH)-related presentations to the EDs in South Western Sydney (SWS). METHODS: We analysed electronic medical records (eMRs) of 7444 MH-related ED encounters of children and young people (CYP) aged up to 18 years from all six public hospitals in SWS from January 2016 to April 2022. Extended LOS was defined as encounters of more than 4 h. We assessed factors associated with extended LOS using a multi-level logistic regression model, accounting for hospital-level clustering. RESULTS: Approximately, 57.6% of all paediatric MH-related ED presentations involved extended LOS. ED presentations by adolescents, patients with a culturally and linguistically diverse background, and those with ambulance arrival had increased odds of extended LOS compared to their counterparts. The odds of extended LOS were lower for encounters that occurred on weekends compared to weekdays, but the odds were higher for presentations that happened at night than during the day. Deliberate self-harm, eating disorder, and schizophrenia spectrum disorders/psychosis-related presentations had higher odds of extended LOS than other MH-related presentations. Patients with MH presentations that required urgent evaluation (triage levels 1-2) had higher odds of extended LOS. Further, the odds of extended LOS were considerably lower during the COVID-19 period compared to the pre-COVID-19 period. CONCLUSION: Our findings highlight the need for equitable distribution of resources directed towards at-risk CYP to improve MH outcomes and reduce health system burden.