Granular and biological activated carbon (GAC and BAC) filters are widely used to remove organic compounds from drinking water sources during municipal drinking water treatment. Common uses of GAC filters include removal of taste and odour (T&O) causing compounds such as geosmin and 2-methylisoborneol and reducing the concentration of dissolved organic matter (DOM) which contributes to the formation of disinfection by-products. GAC filters were installed at the Hamilton Drinking Water Treatment Station (HDWTS) in 2006 to address T&O issues and provide protection against cyanobacterial bloom events, and their effectiveness after such a long operational period was unknown. This study aimed to determine whether bacteria had colonised the GAC surface, compare full-scale GAC filters of different ages in their effectiveness in removing T&O compounds and different fractions of the DOM pool, and assess whether bacterially colonised GAC was able to remove high concentrations of T&O compounds during simulated, transient events after being subjected to steady state influent conditions. Results indicated that bacteria colonise the GAC surface quickly, and that T&O compounds and the humic fraction of the DOM are effectively removed. However, the extent to which humic substances were removed appeared to diminish relatively quickly as filters aged, and the protein fraction of the DOM appeared to be resistant to treatment across the entire treatment train.