Two South Australian drinking water treatment plants (WTPs), Happy Valley and Myponga, with different source water supplies, were selected for an 18-month natural organic matter (NOM) characterisation case study. During this study (January 2001 to July 2002), the raw water dissolved organic carbon (DOC) concentration increased near the end of 2001; however, the applied alum dose required to maintain treated water DOC concentration decreased for both WTPs. This observation linked well with the results obtained from the rapid fractionation. It was found that the increase in organic carbon at the end of the year was mainly an increase of the very hydrophobic acids (VHA) fraction. This fraction typically has the characteristic of being easily removed by alum treatment. The use of the rapid fractionation technique to study the impact of organic character on disinfection was also very successful. The chlorine demand correlated well with the VHA fraction (r2=0.75 for Happy Valley and Myponga combined) while the r2 values of chlorine demand with DOC and UV were 0.55 and 0.58, respectively.