Aim Trastuzumab was first funded in New Zealand for use in HER2+ve stage I–III breast cancer in 2007. This observational study aims to ascertain the patterns of use of trastuzumab in women with invasive HER2+ve breast cancer, and assess the effectiveness of adjuvant trastuzumab in women with stage I–III HER2+ve breast cancer. Methods The Waikato and Auckland Breast Cancer Registries have clinical details of 12 372 women diagnosed with invasive breast cancer between June 2000 and May 2013. The proportion of women with HER2+ve breast cancer treated with trastuzumab was examined by age, ethnicity, stage and year of diagnosis. Differences in outcomes including the development of metastases and death were assessed for women with stage I–III HER2+ve breast cancer treated with both chemotherapy and trastuzumab, compared to women treated with chemotherapy alone. Results Among the 1587 HER2+ve breast cancer patients, 888 (56.0%) women received trastuzumab. The probability of having trastuzumab decreased with higher age and comorbidity score and increased with year of diagnosis, tumor size and cancer stage. Māori and Pacific women were less likely to be treated with trastuzumab. After adjustment for potential confounding factors, the treatment with trastuzumab improved breast cancer-specific mortality (adjusted hazard ratio 0.57, 95% CI: 0.35–0.93). Conclusion Overall, this observational study has shown a substantial improvement in survival for women with HER2+ve stage I–III breast cancer, and much of this improvement can be attributed to the introduction of trastuzumab. Changes in chemotherapy also appear to have led to improved outcomes.