Rates of exoenzyme activity in the water column of 17 sites in the Murray-Darling Basin were determined over a one-year period. The most active exoenzyme was aminopeptidase, followed by alkaline phosphatase, lipase, N-acetyl beta-D-glucoaminidase, alpha-D-glucosidase and beta-D-glucosidase, beta-D-galactosidase and endopeptidase. Exoenzyme activity in rivers (alkaline phosphatase, 0.9-8.2 micromoles/L/day; aminopeptidase, 3.5-32 micromoles/L/day) was within the range reported for Northern-Hemisphere systems, but activity in billabongs (alkaline phosphatase, 4.4-222 micromoles/L/day; aminopeptidase, 8.7-1134 micromoles/L/day) was generally higher than that in rivers and other previously studied systems. Strong relationships were detected between aminopeptidase activity, chlorophyll-a concentrations, bacterial numbers and concentrations of ammonium and dissolved primary amines in the billabong. These interactions were not evident in the river. There were few clear relationships between alkaline phosphatase activity and environmental conditions in either billabong or river.