Social marketing is increasingly being used in the health sector to achieve positive behaviour change. Pacific peoples are a key target audience for most social marketing efforts in New Zealand because of their poor health outcomes and differing conceptualisations of health. Despite this, there is insufficient evidence on the effectiveness and efficacy of social marketing for this population group. There is also little evidence on the validity of behaviour change theories in explaining Pacific peoples’ health behaviour to support improved practice and outcomes. This research explored the effectiveness and efficacy of social marketing for Pacific peoples using a Pacific mixed methods mental models methodology. The validity of the Theory of Planned Behaviour (TPB) in explaining Pacific peoples’ health behaviour was also explored. A qualitative approach was used in the first study to document the evidence-based and culturally relevant critical success factors (CSFs) for effective social marketing for Pacific peoples. A quasi-experimental, mixed methods approach was employed in the second study to ascertain the efficacy of a Pacific social marketing approach based on these CSFs in achieving greater positive behaviour change for Pacific peoples. It was also used to determine the validity of the TPB in explaining this population group’s health behaviour. The results of the first study suggest there are 21 CSFs for Pacific social marketing effectiveness. A Pacific social marketing approach comprising these CSFs was found not to be efficacious in achieving greater positive behaviour change for Pacific peoples in the second study. However, the findings indicate that social marketing effectiveness is associated with ‘ethnicity’, ‘pan-ethnic acculturation’, ‘ethnic-specific acculturation’, the target behaviour and the social marketing approach (messages and stimuli). In addition, the validity of the TPB in explaining Pacific peoples’ health behaviour is only partially supported. This research demonstrates that there is no optimal Pacific social marketing approach for achieving positive behaviour change. Different approaches will be effective for different ethnic-specific groups and for target behaviours. Further research is required to determine the reliability and validity of these findings; and to identify the other factors that influence social marketing efficacy and the validity of the TPB for Pacific peoples.