ObjectivesTo identify sex and age differences in clinically significant symptoms of depression and anxiety and the factors associated with these differences among adults in Australia during COVID-19-related restrictions.DesignAnonymous online survey.SettingAustralia.ParticipantsAdults aged over 18 years living in Australia were eligible and 13 829 contributed complete data. Of these, 13 762 identified as female (10 434) or male (3328) and were included in analyses.InterventionsNone.Outcome measuresClinically significant symptoms of depression (≥10 on Patient Health Questionnaire 9) or anxiety (≥10 on Generalized Anxiety Disorder Scale 7 (GAD-7)), and experiences of irritability (GAD-7 item 6).ResultsWomen were more likely than men to have clinically significant symptoms of depression (26.3% (95% CI 25.4 to 27.1) vs 20.1% (95% CI 18.7 to 21.5), p<0.001) and anxiety (21.8% (95% CI 21.0 to 22.6) vs 14.2% (95% CI 13.0 to 15.4), p<0.001) and to have experienced irritability in the previous fortnight (63.1% (95% CI 62.1 to 64.0) vs 51.4% (95% CI 49.7 to 53.2), p<0.001). They were also more likely than men to be doing unpaid work caring for children (22.8% (95% CI 22.0 to 23.6) vs 8.6% (95% CI 7.7 to 9.6), p<0.001) and dependent relatives (9.8% (95% CI 9.2 to 10.3) vs 5.7% (95% CI 4.9 to 6.5), p<0.001) which made significant contributions to the mental health outcomes of interest. Loss of employment, fear of contracting COVID-19 and feeling a severe impact of the restrictions were associated with poorer mental health in women and men of all ages.ConclusionsRates of clinically significant symptoms of depression and anxiety were higher among women than men. Rather than being intrinsically more vulnerable to mental health problems during the COVID-19 pandemic, the higher risk of symptoms of anxiety and depression among women may in part be explained by their disproportionate burden of unpaid caregiving.