Purpose Post endoscopy upper gastrointestinal cancers (PEUGIC) are potential missed opportunities for the early diagnosis of upper gastrointestinal cancer (UGIC). The primary aim of this study was to define the 3-year rate of PEUGIC across public hospitals in Aotearoa New Zealand (AoNZ), and assess potential root-causes. Methodology A national, retrospective cohort study was performed using data linked between the AoNZ Cancer Registry, Ministry of Health databases, and district-level data. Adult patients diagnosed with UGIC within 6-48 months of gastroscopy from 2010-2022 were included for case-review. Results Five-hundred-and-eighty-three PEUGIC cases were identified from 16/20 health-districts. The unadjusted 3 year PEUGIC rate was 8.3% (95% CI 7.6 to 9.0); 358 cases underwent root-cause analysis (median age 69 years, 31.2% female, 28.8% Barrett’s oesophagus, 10.8% atrophic gastritis). In total, 81/358 cases [22.6%] were interval cancers diagnosed 36-48 months after index endoscopy, while 277 [77.4%] were diagnosed as PEUGIC. PEUGICs most commonly occurred in the lower esophagus/gastroesophageal junction [51.4%] or the stomach lesser curvature [12.2%]. PEUGICs were classified as: A) lesion detected, adequate endoscopy and follow-up, but PEUGIC still occurred [18.8%]; B) lesion detected, inadequate endoscopy or follow-up [43.7%], C) possible missed lesion, adequate endoscopy and follow-up [9.4%], or D) possible missed lesion, inadequate endoscopy or follow-up [28.1%]. 80.5% of PEUGIC were deemed potentially avoidable. Conclusions Opportunities for earlier diagnosis are present in most patients with PEUGIC. Complete endoscopic imaging, systematic sampling, and robust surveillance-planning are highlighted as quality-improvement targets.