Objective: To examine changes in the prevalence of six key chronic disease risk factors (the “Big 6”), from before (2019) to during (2021) the COVID-19 pandemic, among a large and geographically diverse sample of adolescents, and whether differences over time are associated with lockdown status and gender.

Design: Prospective cohort study.

Location: New South Wales, Queensland and Western Australia.

Participants: 983 adolescents drawn from the Health4Life Study.

Primary outcomes measured: The prevalence of physical inactivity, poor diet, poor sleep, excessive recreational screen time, alcohol use, and tobacco use.

Results: The prevalence of excessive recreational screen time, insufficient fruit intake, and alcohol and tobacco use increased over the 2-year period, with alcohol use increasing more among girls. The prevalence of insufficient sleep declined across the full sample; however, increased among girls. The prevalence of high sugar-sweetened beverage and discretionary food consumption reduced among those subjected to stay-at-home orders, compared with those not in lockdown.

Conclusion: Lifestyle risk behaviours, particularly excessive recreational screen time, poor diet, physical inactivity and poor sleep, are prevalent among adolescents. Young people must be supported to find ways to improve or maintain their health, regardless of the course of the pandemic. Targeted approaches to support groups that may be disproportionately impacted, such as adolescent girls, are needed.