Background: The Australian Institute of Health and Welfare has reported an increased rate of hospital-treated intentional self-harm in young females (2000–2012) in Australia. These reported increases arise from institutional data that are acknowledged to underestimate the true rate, although the degree of underestimation is not known. Objective: To consider whether the reported increase in young females’ hospital-treated intentional self-harm is real or artefactual and specify the degree of institutional underestimation. Methods: Averages for age- and gender-standardised event rates for hospital-treated intentional self-harm (national: Australian Institute of Health and Welfare; state: New South Wales Ministry of Health) were compared with sentinel hospital event rates for intentional self-poisoning (Hunter Area Toxicology Service, Calvary Mater Newcastle) in young people (15–24 years) for the period 2000–2012. A time series analysis of the event rates for the sentinel hospital was conducted. Results: The sentinel hospital event rates for young females of 444 per 100,000 were higher than the state (378 per 100,000) and national (331 per 100,000) rates. There was little difference in young male event rates – sentinel unit: 166; state: 166 and national: 153 per 100,000. The sentinel hospital rates showed no change over time for either gender. Conclusion: There was no indication from the sentinel unit data of any increase in rates of intentional self-poisoning for young females. The sentinel and state rates were higher than the national rates, demonstrating the possible magnitude of underestimation of the national data. The reported increases in national rates of hospital-treated self-harm among young females might be due to artefactual factors, such as changes in clinical practice (greater proportion admitted), improved administrative coding of suicidal behaviours or possibly increased hospital presentations of community self-injury cases, but not intentional self-poisoning. A national system of sentinel units is needed for the accurate and timely monitoring of all hospital-treated self-harm.