Background: Hospital-presenting self-harm is a strong predictor of suicide and has substantial human and health service costs. Aims: We aimed to identify changes in case ascertainment after implementation of a new self-harm reporting field at a tertiary hospital in New South Wales, and to report event rates, demographic, and clinical characteristics. Method: Self-harm events presenting to the emergency department (October 2017 to August 2020) were identified using clinical documentation and a new reporting field. Changes in the frequency of self-harm in the period after implementation of the self-harm field were assessed through Poisson regression models. Results: A twofold increase in the frequency of self-harm was detected following the implementation of the new reporting field. The annual average age-standardized event rate of self-harm was 110.4 per 100,000 (120.8 per 100,000 for females; 100.1 per 100,000 for males). The highest rates by age and sex were for females aged 15–19 years (375 per 100,000) and males aged 20–24 years (175 per 100,000). Limitations: Self-harm identification relies on clinician coding practice, which is subject to variability and potential under-enumeration. Conclusion: These findings highlight the value of a self-harm reporting field in hospital record systems for accurate recording and long-term monitoring of self-harm event rates.