The aim of this study was to investigate the effect of various factors, including age, diameter, and slope on the prevenance of eight defect categories in the transmission sewer network of Auckland, New Zealand. A cleaned dataset with the defects identified through recent CCTV inspections of 2817sewers was gathered and linked to a range of physical and environmental factors. Defects were grouped into the following eight categories: gas attack, material damage, infiltration, roots, debris, total joint, structural, and dipped pipe.