Enforcement of legislation restricting retail access to tobacco is increasingly relied on to reduce adolescent smoking rates. In 1996, health authorities in the Northern Sydney Health Area began monitoring tobacco retailer compliance (PROOF program) with staged purchase attempts by adolescents below the legal age (18 years). Repeat cross-sectional surveys before (1995) and after (2000) the introduction of PROOF monitored changes in adolescent smoking behaviour. Students aged 12 to 17 years from 11 Northern Sydney metropolitan public secondary schools were surveyed for self-reported smoking and tobacco purchasing behavior in 1995 (n = 5,206) and 2000 (n = 4,120). Between 1996 and 2000, 545 retailer compliance checks found 34% unlawfully sold cigarettes to minors and 28% of these repeated the offence. Nine prosecutions resulted. Modelling revealed a significant association between the intervention and never having smoked (adjusted OR = 1.16, 95% CI = 1.01-1.33) although there was no significant association with being a current smoker. The odds of being a smoker were greater for students from coeducational schools, with this effect being modified by gender. There was no reduction in adolescent smoking with active enforcement of tobacco access laws despite an apparent increase in students who reported never to have smoked.