Childhood obesity monitoring is a fundamental component of obesity prevention but is poorly done in Australia. Monitoring obesity prevalence in children provides important population health data that can be used to track trends over time, identify areas at greatest risk of obesity, determine the effectiveness of interventions and policies, raise awareness and stimulate action. High participation rates are essential for effective monitoring because these provide more representative data. Passive ('opt-out') consent has been shown to provide high participation rates in international childhood obesity monitoring programs and in a recent Australian federal initiative monitoring early child development. A federal initiative structured like existing child development monitoring programs, but with the authority to collect height and weight measurements using opt-out consent, is recommended to monitor rates of childhood obesity in Australia.