Urban riparian zones are highly valuable for a range of environmental, social and economic reasons. Despite their ecological complexity, legislative instruments underpinning riparian protection focus predominantly on minimum corridor widths required to fulfill environmental functions. This study sought to identify the dominant environmental variables that influence riparian corridor biodiversity in northern Sydney, Australia. Ant and plant assemblages were sampled from 18 riparian corridors of various widths. Environmental data pertaining to each corridor were collected using field and desktop surveys. Generalised Linear Modelling found perimeter:area ratio and the gradient of the slope perpendicular to the stream to predict significantly ant and plant richness. Plant richness was also influenced by vegetation community type and corridor width. When streamside vegetation communities were considered in isolation, many of the same factors were found to influence species richness. Likewise, Distance-based Linear Modelling identified vegetation community type and landscape connectivity as significant predictors of ant composition both in streamside habitats and those located across the full width of each riparian corridor sampled. While width was the best predictor of plant assemblage composition across the whole riparian corridor, streamside vegetation was influenced more significantly by soil pH. This result arose principally due to a dominance of exotic plants in alkaline soils. Although wider corridors were likely to encompass a greater variety of biotic assemblages, landscape and site scale environmental characteristics appeared to be of greater ecological significance. Environmental managers should therefore consider these factors and target weed invasion when seeking to enhance biodiversity in urban riparian corridors.