To prevent deterioration of paediatric mental health during a pandemic, it is important to understand the various factors that may impact mental health outcomes. This study examined which vulnerability factors predicted poorer anxiety and depression symptoms in Australian children and youth (aged 4-17 years) during the initial lockdown period (May – August 2020), and whether outcomes varied according to gender or age. A parent-report questionnaire was used to investigate parent-related vulnerability factors including parent mental health, family accommodation behaviours, and COVID-19-related worries, and child-related factors including fears about illness/contamination, fears about social distancing, behaviour modification, and virus impact. Data was analysed using four multiple regression models. Greater parent stress and family accommodation predicted increased child anxiety symptoms, whilst family accommodation predicted increased child depression symptoms. Regarding child-specific vulnerability factors, perceived virus impact predicted increased anxiety and depression symptoms. Greater social-distancing fears predicted depression symptoms, but not anxiety symptoms.