We set out to improve understanding of the effectiveness of streamside management zones (SMZs) for protecting water quality in landscapes dominated by agriculture. We conducted a paired-catchment experiment that included water quality monitoring before and after the establishment of a forest plantation as an SMZ on cleared farmland that was used for extensive grazing. In a second study, we monitored water quality during the harvesting of a 20-year-old plantation in an SMZ. We found concentrations of bacteria, sediment and phosphate were lower in the buffered paired catchment, but that lower nitrogen concentrations could not be attributed to the intervention. Harvesting caused no appreciable increase in sediment delivery to the stream and we found it to be a minor source compared to other disturbances (road drainage and cattle disturbance).