The aim of the study was to explore the associations between obesity, weight perceptions and gender, ethnicity, culture and social class in a large national study of Australian school children. Primary and high schools (N = 47) were recruited from every state and territory of Australia and included 7889 children from government, private and Catholic schools (82% response rate) in August-November, 2006. The socioeconomic status (SES) of schools was based on a government survey of total family income. A questionnaire completed by students, measured demographic details of gender, age, weight perceptions and ethnic/cultural background. Height and weight were measured by trained research assistants. Outcome measures included body mass index (BMI), prevalence of obesity, overweight, weight perceptions. Prevalence of obesity was 6.4% of males and 5.6% of females in primary school students (P = 0.34). More high school males were obese than females (7.7% vs. 5.7%, P = 0.001). Obesity was more prevalent among students from Pacific Islander backgrounds. Adolescents who were most likely to be obese were boys and girls of low SES or Pacific Islander or Middle Eastern/Arabic background. The least likely to be obese were Anglo/Caucasian or Asian students and in particular, the girls. Obese female adolescents from Aboriginal, Middle Eastern/Arabic and Pacific Islander backgrounds were less likely than their Caucasian or Asian peers to perceive themselves as 'too fat'.Those working in clinical, community or educational settings with young people and in particular, obese young people, should be aware that obesity is likely to be more prevalent, more culturally acceptable and perhaps more desirable among children and teens from low SES communities and/or Middle Eastern and Pacific Islander backgrounds. Health and social work professionals should be careful not to exaggerate the risks of overweight or obesity or inadvertently create weight concerns among young people. The different body image perceptions identified in this study should be taken into account when planning clinical, community or preventive initiatives among children or adolescents from varying ethnic groups.