One of the current concerns for water treatment plants using alum as a coagulant is elevated concentration of residual aluminium in finished water. The objective of the study is to examine the seasonal variations and factors influencing residual aluminium at the Buffalo Pound Water Treatment Plant (BPWTP). Analysis of BPWTP data for two years related to seasonal variations of total/dissolved aluminium levels showed that much of the raw water total aluminium were in particulate form. Raw water particulate aluminium is correlated well (r2 = 0.79 for 1996 and r2 = 0.82 for 1997) with raw water total suspended solids indicating much of the particulate aluminium is derived from the suspended solids present in raw water. An analysis of eight-year data on total/dissolved aluminium, turbidity, dissolved organic carbon and applied alum dosage showed that dissolved organic carbon present in the raw water played a major role in controlling efficacy of alum coagulation at BPWTP. The data showed that when alum/DOC ratio is less than 7, insufficient alum addition led to incomplete coagulation resulting in colloidal material mostly consisting of organic aluminium in particulate form. Hence particulate aluminium increased in treated water. But this increase in particulate aluminium did not increase the turbidity of treated water. This indicated that an adequate alum dose in response to dissolved organic carbon change is important in minimising residual aluminium in treated water at BPWTP.