Background/Objective: To report 30-year (1985-2015) prevalence trends in overweight, obesity and abdominal obesity among children by school level and socioeconomic status (SES). Subjects/Methods: Five cross-sectional, population child surveys (age 4-18 years; n=27 808) conducted in 1985-1997-2004-2010-2015 in New South Wales, Australia. Outcomes were prevalence of measured overweight, obesity and waist-to-height ratio (WHtR≥0.5) by sex, school level (children (primary) and adolescents (high)) and SES tertile. Results: In 2015, the prevalences of overweight, obesity and WHtR≥0.5 in children were 16.4%, 7.0% and 14.6%, respectively, and in adolescents 21.9%, 17.2% and 4.6%, respectively. Obesity prevalence has not significantly changed in children or adolescents since 1997, nor since 2010 (children, P=0.681; adolescents, P=0.21). Overweight has not significantly changed in children since 1997, but has in adolescents since 1985, with a relative increase of 16 percentage points (P<0.001) between 2010 and 2015. WHtR≥0.5 prevalence has significantly changed since 1985, except in adolescent girls between 2010 and 2015. Between 2010 and 2015 the relative increase in WHtR≥0.5 was 17 and 40 percentage points in children and adolescent boys, respectively. Significant disparities in prevalence rates between children and adolescents from low and high SES backgrounds began in 2010 for overweight, since 1997 for obesity and since 2004 for WHtR≥0.5. Differences between SES groups have become larger over the past 18 years. Conclusions: Since 1997, obesity has remained stable, and overweight has stabilized in children, not in adolescents. WHtR≥0.5 significantly increased between 1985 and 2015, with prevalence rates at each survey around twice the obesity prevalence. Compared with high SES children and adolescents, the risk of overweight, obesity and WHtR≥0.5 was significantly higher for low SES children and adolescents. The findings are highly relevant to policy makers involved in child obesity prevention interventions and highlight the need for better targeted interventions among children and adolescents from low SES backgrounds, and adolescents in particular.