Background Acute mastoiditis (AM) is a complication of severe or poorly treated acute otitis media (AOM) in children. The impact of the ‘immunity debt’ phenomenon associated with COVID-19 pandemic restrictions has yet to be assessed in the Australian health-care setting in relation to AM. Methods A retrospective chart review was performed on paediatric patients (<18 years) diagnosed with AM in a tertiary paediatric hospital from January 2012 to June 2025. Patients were categorised into time periods: pre-COVID, COVID and post-COVID based on Australian health policy timelines. Baseline demographic and admission data were collected. The primary outcomes were AM incidence per emergency department admissions, mastoiditis complications and disease severity, surgical intervention and microbial spectrum. A sub-analysis comparing pre-COVID and a combined "immunity debt period" (COVID and post-COVID) was performed. Results 176 patients were included between 1/1/12 and 30/6/25. AM incidence sharply increased in the post-COVID era and exceeded stable pre-pandemic levels. The proportion of patients with intracranial complications increased from pre-, COVID, and post-COVID periods. Mastoidectomy rates doubled from 13.43% to 28.40% post-pandemic while MEVT rates were unchanged. Streptococcus anginosus was increasingly identified in post-COVID cases and all had intracranial complications and mastoidectomy. Sub-analysis identified a higher rate of very severe disease in the immunity debt period compared to pre-COVID (p=0.018). Conclusion Immunity debt may be contributing to the increased incidence and severity of paediatric AM in the post-pandemic setting. Greater suspicion of intracranial complications and rapid escalation of therapy are recommended for the management of paediatric AM in the post-pandemic setting.