As part of the CarbonWatch-NZ research programme, air samples were collected at 28 sites around Auckland, New Zealand to determine the atmospheric ratio (R_CO) of excess (local enhancement over background) carbon monoxide to fossil CO₂ (CO₂ff). Sites were categorised into seven types (background, forest, industrial, suburban, urban, downwind, and motorway) to observe R_CO around Auckland. Flasks from motorway sites observed R_CO of 14 ± 1 ppb/ppm and were used to evaluate traffic R_CO. The similarity between suburban (14 ± 1 ppb/ppm) and traffic R_CO suggests that traffic dominates suburban CO₂ff emissions during daytime hours, the period of flask collection. The lower urban R_CO (11 ± 1 ppb/ppm) suggests that urban CO₂ff emissions are comprised of more than just traffic, with contributions from residential, commercial, and industrial sources, all with a lower R_CO than traffic. Finally, the downwind sites were believed to best represent R_CO for Auckland City overall (11 ± 1 ppb/ppm). We demonstrate that the initial discrepancy between the downwind R_CO and Auckland’s estimated daytime inventory R_CO (15 ppb/ppm) can be attributed to an overestimation in inventory traffic CO emissions. After revision based on our observed motorway R_CO, the revised inventory R_CO (12 ppb/ppm) is consistent with our observations.