The effects of New Zealand freshwater crayfish or koura (Paranephrops planifrons:Parastacidae) on organic matter processing, sediment accumulation, and benthic invertebrate communities were investigated using four replicate treatments of 0 (control), 4–5 (medium), and 8–11 (high) similar‐sized koura in 0.5 m2artificial stream channels colonised by benthic invertebrates from a pasture stream, Waikato, New Zealand. Wineberry (Aristotelia serrata) leaf packs were placed in each channel and after 7 weeks the leaf matter remaining was significantly lower in both medium and high koura channels than in controls. The amount of sediment (surficial cover by fines and weight of suspendable sediment) was also significantly reduced in high koura density channels. Densities of invertebrates other than crayfish were not significantly different among treatments; however, taxa richness and invertebrate biomass were significantly lower in high koura channels than in controls. Our results suggest that freshwater koura may play a keystone role in structuring benthic invertebrate communities either directly through predation, or indirectly by sediment bioturbation and increasing organic matter processing rates.