Background: Surveillance following surgery for colorectal cancer aims to detect treatable disease relapse or metachronous neoplasia. Metachronous cancers have been reported within a short duration of follow-up, and may be due to missed lesions, seeding into polypectomy wounds or accelerated tumorigenesis related to genetic instability. The purpose of this study was to establish the timing and method of detection of metachronous cancers in a large population of patients in a surveillance database. Methods: This retrospective clinical study used patients with an elevated risk of colorectal neoplasia included in a colonoscopy-based surveillance programme to identify those with two or more colorectal cancers, as well as the timing and method of detection of the tumours. Colonoscopy reports and histopathology results were reviewed to determine quality of bowel preparation, tumour location, and polypectomy data. Results: Fourteen (2.5%) of 569 patients with colorectal cancer developed metachronous malignant tumours, nearly half of which were identified within 3 years of follow-up by surveillance colonoscopy or an interval faecal immunochemical test for globin. None of these had a previous polypectomy at the site of the second tumour, bowel preparation at the original colonoscopy was good in most cases, and no metachronous tumour occurred at a colonic flexure. Conclusion: Metachronous cancers can occur early during follow-up after curative intent resection, and early colonoscopic surveillance may be warranted.