Background Australian immigration policy resulted in large numbers of children being held in locked detention. We examined the physical and mental health of children and families who experienced immigration detention. Methods Retrospective audit of medical records of children exposed to immigration detention attending the Royal Children’s Hospital Immigrant Health Service, Melbourne, Australia, from January 2012 –December 2021. We extracted data on demographics, detention duration and location, symptoms, physical and mental health diagnoses and care provided. Results 277 children had directly (n = 239) or indirectly via parents (n = 38) experienced locked detention, including 79 children in families detained on Nauru or Manus Island. Of 239 detained children, 31 were infants born in locked detention. Median duration of locked detention was 12 months (IQR 5–19 months). Children were detained on Nauru/Manus Island (n = 47/239) for a median of 51 (IQR 29–60) months compared to 7 (IQR 4–16) months for those held in Australia/Australian territories (n = 192/239). Overall, 60% (167/ 277) of children had a nutritional deficiency, and 75% (207/277) had a concern relating to development, including 10% (27/277) with autism spectrum disorder and 9% (26/277) with intellectual disability. 62% (171/277) children had mental health concerns, including anxiety, depression and behavioural disturbances and 54% (150/277) had parents with mental illness. Children and parents detained on Nauru had a significantly higher prevalence of all mental health concerns compared with those held in Australian detention centres. Conclusion This study provides clinical evidence of adverse impacts of held detention on children’s physical and mental health and wellbeing. Policymakers must recognise the consequences of detention, and avoid detaining children and families. Copyright: © 2023 Tosif et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.