Objective:As a result of the coronavirus-19 disease (COVID-19) pandemic, Australia adopted emergency measures on 22 March 2020. This study reports the effect of the COVID-19 lockdown on appetite and overeating in Australian adults during the first month of emergency measures.Design:This study reports analysis of data from population-based, self-completed survey. The main outcome measure was an item from the Patient Health Questionnaire 9 (PHQ-9) asking: "Over the past 2 weeks, how often have you been bothered by poor appetite or overeating?". Data on sociodemographic factors, symptoms of anxiety and depression, and the impact of COVID-19 and lockdown were also collected. Multivariable logistic regression was used to examine associations with poor appetite or overeating.Setting:An anonymous online survey available from 3 April to 2 May 2020.Participants:A total of 13,829 Australian residents aged 18 years or over.Results:The weighted prevalence of being bothered by poor appetite or overeating in the past two weeks was 53.6%, with 11.6% [95% confidence interval (CI): 10.6-12.6] of the cohort reporting poor appetite or overeating nearly every day. High levels of anxiety, concern about contracting COVID-19, being in lockdown with children and reporting a severe impact of the lockdown were associated with increased odds of poor appetite or overeating.Conclusions:Given the widespread prevalence of being bothered by poor appetite or overeating, universal public health interventions to address emotion-focussed or situational eating during periods of lockdown may be appropriate.