Amplicon sequence fingerprinting of communities in activated sludge systems have provided data revealing the true level of their microbial biodiversity and led to suggestions of which intrinsic and extrinsic parameters might affect the dynamics of community assemblage. Most studies have been performed in China and Denmark, and comparatively little information is available for plants in other countries. This study looked at how the communities of three plants in Victoria, Australia, treating domestic sewage changed with season. All were designed to remove nitrogen microbiologically. They were all located close together to minimise any influence that climate and demographics might have on their operation, and samples were taken at weekly intervals for 12 months. 16S rRNA amplicon sequencing revealed that each plant community was distinctively different to the others and changed over the 12-month sampling period. Many of the factors suggested in other similar studies to be important in determining community composition in activated sludge systems could not explain the changes noted here. The most likely influential factors were considered to be temperature and influent composition reflecting changes in dietary intake by the populations served by each plant, since in all three, the most noticeable changes corresponded to seasonal shifts. KEY POINTS: • Monitoring microbial communities in 3 wastewater treatment plants removing nitrogen • Temperature is the most influential factor in dynamic changes in community composition.