The COVID-19 pandemic presents significant risks to the mental health and wellbeing of families. This study aimed to examine: (1) patterns of parent and child (0–18 years) mental health, parent substance use, couple conflict, parenting practices, and family functioning during COVID-19 compared to pre-pandemic data; and (2) associations between parent, child, and family outcomes during the pandemic and both pre-existing risk factors and COVID-19 stressors. Participants were Australian mothers (81%) and fathers aged 18 years and over who were parents of a child 0–18 years (N=2,365). Parents completed an online self-report survey assessing mental health, substance use, couple conflict, parenting, and family functioning during ‘stage three’ COVID-19 restrictions in April 2020. Data were compared to pre-pandemic data from four Australian population-based cohorts. Compared to pre-pandemic estimates, during the pandemic period parents reported higher rates of parent mental health symptoms (Cohen’s d=0.26-.81, all p<.001), higher parenting irritability (d=0.17-.46, all p<.001), lower family positive expressiveness (d=-0.18, p<.001), and higher alcohol consumption (22% vs 12% drinking four or more days per week, p<.001). In multivariable analyses, pre-existing financial deprivation and COVID-19 stressors were associated with greater severity in parent and child mental health symptoms, parent emotion dysregulation, parenting irritability, couple conflict and family positive/negative expressiveness. Parents and children with pre-existing mental health conditions had elevated difficulties during the pandemic across most domains. Our data suggest wide-ranging, detrimental family impacts associated with the COVID-19 pandemic; and support policy actions to assist families with financial supports, leave entitlements, and social housing.