Abstract Purpose Population based cancer registries are an invaluable resource for monitoring incidence and mortality for many types of cancer. Research and healthcare decisions based on cancer registry data rely on the case completeness and accuracy of recorded data. This study was aimed at assessing completeness and accuracy of breast cancer staging data in the New Zealand Cancer Registry (NZCR) against a regional breast cancer register. Methodology Data from 2562 women diagnosed with invasive primary breast cancer between 1999 and 2011 included in the Waikato Breast Cancer Register (WBCR) were used to audit data held on the same individuals by the NZCR. WBCR data were treated as the benchmark. Results Of 2562 cancers, 315(12.3%) were unstaged in the NZCR. For cancers with a known stage in the NZCR, staging accuracy was 94.4%. Lower staging accuracies of 74% and 84% were noted for metastatic and locally invasive (involving skin or chest wall) cancers, respectively, compared with localized (97%) and lymph node positive (94%) cancers. Older age (>80 years), not undergoing therapeutic surgery and higher comorbidity score were significantly (p<0.01) associated with unstaged cancer. The high proportion of unstaged cancer in the NZCR was noted to have led to an underestimation of the true incidence of metastatic breast cancer by 21%. Underestimation of metastatic cancer was greater for Māori (29.5%) than for NZ European (20.6%) women. Overall 5-year survival rate for unstaged cancer (NZCR) was 55.9%, which was worse than the 5-year survival rate for regional (77.3%), but better than metastatic (12.9%) disease. Conclusions Unstaged cancer and accuracy of cancer staging in the NZCR are major sources of bias for the NZCR based research. Improving completeness and accuracy of staging data and increasing the rate of TNM cancer stage recording are identified as priorities for strengthening the usefulness of the NZCR.