Purpose To define the 3-year rate of post colonoscopy colorectal cancers (PCCRC) across public hospitals in Aotearoa New Zealand (AoNZ), with a root-cause analysis. Methodology A national, retrospective cohort study was performed using linked-data linked from the AoNZ Cancer Registry, Ministry of Health databases, and local district data. Adult patients diagnosed with CRC 6-48 months following a colonoscopy between 2010 and 2022 were included. World Endoscopy Organisation classifications for PCCRC were applied on case-review: (A) possible missed lesions, prior exam adequate; (B) possible missed lesions, prior exam inadequate; (C) detected lesions, not resected; or (D) likely incomplete resection of previously identified lesions. Results A total of 1702 cases were identified across 16/20 districts in AoNZ, covering >90% of the population. The unadjusted PCCRC-3 year rate was 5.9% (95% CI 5.6-6.2). Nine-hundred-and-fifty-five cases (56.1%, median age 75 years, 50.9% female) underwent root cause analysis; this revealed 423 category A [44.3%], 364 category B [38.1%], 29 category C [3.0%] and 139 category D PCCRCs [14.6%]. PCCRCs were most likely to occur in the caecum [15.1%], ascending colon [15.0%] or rectum [20.4%]. Potential missed lesions (category A/B) were more likely to occur in high-risk patients (history of inflammatory bowel disease, CRC, or hereditary cancer syndrome; 41.3% vs 23.2%, p<0.001). 90.6% of cases were deemed potentially avoidable. Conclusions Most PCCRCs originate from potentially missed, or incompletely resected lesions, and commonly involve the rectum or proximal colon. This national dataset provides the foundation for ongoing quality improvement initiatives to minimise PCCRC in AoNZ.