Abstract Background Community-based obesity treatment programs have become an important response to address child obesity; however the majority of these programs are small, efficacy trials, few are translated into real-world situations (i.e., dissemination trials). Here we report the short-term impact of a scaled-up, community-based obesity treatment program on children’s weight and weight-related behaviours disseminated under real world conditions. Methods Children age 6–15 years with a body mass index (BMI) ≥85th percentile with no co-morbidities, and their parents/carers participated in a twice weekly, 10-week after-school child obesity treatment program between 2009 and 2012. Outcome information included measures of weight and weight-related behaviours. Analyses were adjusted for clustering and socio-demographic variables. Results Overall, 2,812 children participated (54.2 % girls; M age 10.1 (2.0) years; M attaendance 12.9 (5.9) sessions). Beneficial changes among all children included BMI (−0.65 kg/m 2 ), BMI-z-score (−0.11), waist circumference (−1.8 cm), and WtHtr (−0.02); self-esteem (+2.7units), physical activity (+1.2 days/week), screen time (−4.8 h/week), and unhealthy foods index (−2.4units) (all p