Objective: COVID-19 has led to disruptions to the lives of Australian families through social distancing, school closures, a temporary move to home-based online learning, and effective lockdown. Understanding the effects on youth mental health is crucial to inform policies to support communities as they face the pandemic and future crises. This paper sought to report on mental health symptoms in Australian children and adolescents during the initial stages of the pandemic (May to November 2020) and to examine its association with child/family characteristics and exposure to the broad COVID-19 environment. Methods: An online longitudinal survey was completed by 1,324 parents and carers of Australian children aged 4 to 17 years. Parents/carers reported on their child’s mental health using five measures, including emotional symptoms, conduct disorder problems, hyperactivity/inattention, anxiety symptoms and depressive symptoms. Child/family characteristics and COVID-related variables were measured. Results: Overall, 30.5%, 26.3% and 9.5% of our sample scored in the high to very high range for emotional problems, conduct problems and hyperactivity/inattention, respectively. Similarly, 20.2% and 20.4% of our sample scored in the clinical range for anxiety symptoms and depressive symptoms respectively. A child’s pre-existing mental health diagnosis, neurodevelopmental condition and chronic illness significantly predicted parent-reported youth mental health symptoms. Parental mental health symptoms, having a close contact with COVID-19 and applying for government financial assistance during COVID-19 were also significant predictors of youth mental health symptoms. Conclusions: Our findings show that Australian youth experienced considerable levels of mental health symptoms during the initial phase of COVID-19, and highlight the need for targeted, effective support for affected youth and particularly for those with pre-existing vulnerabilities