Constructed stormwater wetlands are one strategy to mitigate the impact of urbanisation on aquatic ecosystems. However, the biotic community is generally dominated by organisms that are able to tolerate poor water quality. The reduced macroinvertebrate diversity and abundance in comparison to natural wetlands, and prevalence of invasive species such as Gambusia, can influence the flow of energy through food webs. We used stable isotope analysis (δ13C and δ15N) of food webs to determine whether the amount of catchment urbanisation (measured as total imperviousness, the percentage of a catchment covered in impervious surfaces) influenced basal resources and trophic relationships in constructed wetlands, in Melbourne, Australia. We found that as total imperviousness (TI) increased there was a significant decrease in the abundance and diversity of macroinvertebrates and a significant increase in the values of δ13C and δ15N recorded for fishes and macroinvertebrates. An increase in TI was associated with a decrease in the mean trophic position of fishes and an increase in the mean trophic position of macroinvertebrates. Our results suggest that sources of carbon differed between sites of low and high imperviousness and that nitrogen sources increased with increasing TI. This study provides an understanding of the likely consequences of disturbance associated with urbanisation on the food-web structure of constructed wetlands.