The North Island Robin (Petroica longipes) is an endemic New Zealand passerine with melanin-based plumage of grey-brown and black, with males typically described as having darker plumage than females. In this study we quantified the relationship of sex and age on plumage colour in 32 North Island Robins on Tiritiri Matangi Island, New Zealand. We objectively scored plumage colour in each individual relative to a colour identity chart created with reference to HTML code. Our results support the general assertion that males are darker than females in this species. However, we found that this is significantly confounded by the age of the bird in both sexes and that this relationship is not a simple dichotomy between first-year breeders and adults. This is the first study to document the existence of delayed plumage maturation in the female of this species, and that plumage changes occur over many years in both sexes. The plumage of most Robins lies within the colour-range overlap between the sexes, indicating that plumage colour is not a reliable criterion for sexing this species. We used a classification and regression tree (CART) analysis to determine which morphometric measures were the best classifiers of gender in birds within the colour-range overlap; a tarsus measure cut-off of ~36 mm accurately predicted gender in >80% of cases. We propose that delayed plumage maturation in this species results from social and physiological factors during moult, which affect melanin production and the colouring of the feathers.