Lockdowns were used worldwide to mitigate the spread of SARS-CoV-2. We demonstrate that the 112-day hard lockdown in Melbourne, Australia, the longest among OECD jurisdictions, exclusively penalized families with young children. To identify the causal impact of lockdown, we interrogated nationally-representative longitudinal survey data and exploited quasi-experimental variation in Melbourne's lockdown, one that left other jurisdictions unaffected. Using difference-in-differences estimation, we found that, surprisingly, most vulnerable groups (the young, poor, lonely and those with previous mental health conditions) were left unscathed. However, we found mothers experienced significant, sizable declines in health and work hours, and increases in loneliness, despite feeling safer and being more active. Zero-COVID policies are not as harmful as may have been expected but came at high cost to mothers in society.