Objective: To explore population-level trends in health behaviours and wellbeing indicators from before (2017–2019) to during (2020–2022) the COVID-19 pandemic in Australia. Methods: Using cross-sectional data from New South Wales Adult Population Health Surveys (2017–2022, n = 73,680 responses), we calculated weighted prevalence estimates and interrupted time-series logit models to investigate trends in health behavioural risk factors (vegetable and fruit intake, physical activity, alcohol consumption, smoking, e-cigarette use), Body Mass Index (BMI) (overweight and obesity), and wellbeing indicators (psychological distress, self-rated health) among adults aged ≥ 16 years before and during the pandemic. Results: From 2017 to 2022, the behavioural risk factors and BMI trends were mostly unchanged. Similarly, wellbeing indicator trends showed only minor variations according to age. The interrupted time-series models found marked changes from before to during the COVID-19 pandemic for e-cigarette use and self-rated health. E-cigarette use showed an overall increasing trend, with significant increases from 2017 to 2022 (OR 8.25, 95 %CI 6.10–11.16). Poor self-rated health showed a stable trend before COVID-19, but decreased in 2020 (OR 0.68, 95 %CI 0.58–0.80) and 2021 (OR 0.70, 95 %CI 0.60–0.81), returning to pre-COVID levels in 2022 (OR 1.23, 95 %CI 1.07–1.41). During the pandemic (2020–2022), there were few statistically significant observed changes in prevalence trends according to SES indicators. Conclusion: Among Australian adults, relatively small population-level impacts of the COVID-19 pandemic on health behaviours and wellbeing trends were observed. Continued surveillance and sub-group analyses are essential for investigating potential time-lagged effects and regional or sociodemographic differences in health behaviours and wellbeing.