Streams are fuelled by microbial decomposition of leaf litter from riparian vegetation. However, urbanisation has altered the quality and quantity of leaf litter sources to streams (i.e. native vs exotic plant species). Here we aimed to determine 1) the formation of microbial biofilms on exotic versus native leaves, 2) how biofilm community structure relates to nutrient release from leaf litter, and 3) how decomposition of litter is effected by water nutrient levels. Single and mixed treatments of Eucalyptus camadulensis and Salix ferrisa leaves were incubated in three water types (undisturbed catchment - low in nutrients, urbanised catchment - high in nutrients, and deionised water - control), under controlled light and temperature conditions for seven days. 16S rRNA and ITS sequencing revealed strong differences in bacterial and fungal communities between exotic and native leaves, and this was dependant on the nutrient concentration of stream water. E. camadulensis was associated with polyphenol decomposing bacteria and greater relative abundance of fungi. Further, FISH-CSLM indicated that fungi likely assists bacteria in the colonisation and decomposition of leaf litter, particularly for E. camadulensis. Initial stream water chemistry (SRP, DIN, DOC) and composition of leaf leachates appear to drive bacterial community structure and function, likely owing to C:N:P limitation and bioavailability, which could impact ecosystem function on a wider scale. E. camadulensis and S. ferrisa are known to be either invasive or native to riparian communities in both the northern and southern hemispheres, thus these findings have global implications for stream management.