External conditions can have a major effect on chemical condition and biota in aquatic systems. When long term dry conditions are broken by large rain events the associated nutrient and organic matter inflow a lake can have cause significant changes. Canberra experienced an extremely dry period prior to January 2020. Large rainfall events in January and February of 2020 broke the dry spell. This caused External nutrient inputs to the lake after mid-January caused large increases to in-lake nutrient loads. This included a doubling and six-fold increase of the of the in-lake total phosphorus and dissolved reactive phosphorus loads respectively. These changes in turn affected the biovolume and community composition of cyanobacteria in the lake. Up until mid-January Aphanocapsa delicatissima was the dominant species of cyanobacteria in the lake. After mid-January there was an increase in cyanobacteria biovolume in the lake and a mixture of heterocyst forming species including Sphaerospermopsis aphanizomenoides, Dolichospermum spiroides, Dolichosprermum planktonicum and Dolichospermum circinale dominated cyanobacteria biomass in the lake. The changes in the lake demonstrate the importance of rainfall driven external nutrient loads in supporting cyanobacteria blooms in the lake and determining community composition.