The COVID-19 lockdown has had an immediate impact on obesity rates among 4-year-old children, according to study utilising growth data from the B4 School Check information system. Data were reviewed from 1 July 2012 to 30 June 2022, with 25 March 2020 used as the index date for comparison of pre- and post-COVID periods. The overall rate of obesity increased by 1.8% after COVID-19, and by 2% among males. When analysed by ethnic group, Pasifika individuals had the greatest increase in obesity rate (4.3%), followed by Māori (2.2%). Increases in obesity rates were greater in children from the most deprived areas (3%) and in those from the Auckland region (2.5%).