The frequency and intensity of extreme wildfire events in forested catchments are increasing globally. However, little is understood about the immediate and direct consequences of extreme wildfires on stream ecosystems due to the unpredictable nature of the events. There is some evidence that during the combustion phase of extreme wildfire events, acute short-term stream temperature spikes may occur and impact the stream's physicochemical conditions and the stream invertebrate assemblages within them. Mesocosm experiments that simulated wildfire-induced temperature spikes on natural stream invertebrate assemblage were completed to identify the potential effects on abundance, community composition, and functional feeding groups. Twelve repeated measures laboratory stream experiments were run in an adapted Before-After-Control-Impact-Pair design to analyse the influence of water temperature spikes of 45°C and 55°C. When exposed to a spike of 55°C, an average reduction of 95.9% (SD±3.25) of in-stream invertebrate abundance was measured, significantly lower (p=0.001) than the Control. Functional feeding group diversity was also significantly impacted and reduced to only shredders and scrapers remaining. The changes seen in functional feeding groups may impact recovery post-fire due to the loss of ecological function. Individual families responded differently, with some being very sensitive to the increase in temperature and two being tolerant of the spike. In contrast, the spike to 45°C had no significant impact (p=0.311) on the abundance or diversity of functional feeding groups. This study highlights the detrimental effects that water temperature spikes associated with extreme wildfire events may have on stream invertebrate communities and their ecological functions.