Levels of participation in physical activity and sport by the New Zealand population are in decline, whilst the levels of sedentary behaviours are rising. Developing positive physical activity experiences, perceptions and habits in childhood may provide an effective approach to decrease the burden of inactivity. The purpose of this thesis was to improve knowledge of the efficacy of a sport-based intervention to increase physical activity levels of New Zealand children from a low socio-economic background. This was achieved by implementing an after-school soccer intervention at two low decile schools in Auckland, New Zealand. To determine current levels of physical activity, fifty-eight children wore a NL-2000 pedometer for four consecutive days (three weekdays and one weekend). Mean step counts (± SD) for boys were 17018 (± 4640) and for girls 12415 (± 4329) on weekdays, and for boys 12507 (± 4338) and girls 9537 (± 4421) on weekends. Nearly 50% of girls and 37% of boys were not reaching previously published daily step count recommendations of 15,000 for boys and 12,000 for girls during weekdays. The feasibility and efficacy of a six-week after-school soccer programme (2/hr.wk-1) on physical activity levels of 70 children (43 boys, 27 girls) compared to a control group of 25 children (23 boys, 2 girls) was determined in a randomised controlled trial. Measures of physical activity (4 day sealed pedometry), mass and height were completed at baseline, Week 6 (end of the intervention), and at three-month post-intervention. Compared to control, participants in the soccer programme attained higher weekday step counts after 6 weeks (treatment 16980 ± 4515; control 15021 ± 3783) and these were sustained three months post-intervention (treatment 16218 ± 4591; control 14591 ± 3488). However, these step count differences were not statistically significant. When children were grouped into activity tertiles (low, moderate and highly active) the intervention effect was more evident in the low to moderately active children. Further analysis revealed that the treatment groups’ moderate activity tertile was significantly more active than the control at follow up (p = 0.0399). This programme may offer a viable alternative to traditional physical activity interventions which concentrate on other forms of physical activity accumulation such as active transport and physical education. However, additional research needs to be carried out to determine whether the absence of statistical differences is simply a lack of statistical power.