Herbicide use varies both spatially and temporally within managed forests. While information exists on the effects of herbicide use on water quality at the site and small catchment scale, little is known about the cumulative effects of herbicide use at the landscape scale. A cumulative effects analysis was conducted in the upper Rangitaiki catchment (118,345 ha) in New Zealand, to determine the risk to water quality from two herbicides commonly used for post-plant weed control: terbuthylazine and hexazinone. Data from a 12.5 ha catchment study within the Rangitaiki Basin, where these herbicides were aerially applied in two consecutive years, were used to model herbicide concentrations entering the Rangitaiki River for the remaining treated areas in the catchment for the same two spray seasons. A spreadsheet model routed herbicide residues from their point of entry into the river to a flow gauge on the Rangitaiki River. Modeling of chemical residue loadings and concentrations of terbuthylazine and hexazinone indicated that potential herbicide residues in stream waters would be mainly below analytical detection limits and pose no risk to the aquatic environment or human health and safety. Safety factors for drinking water quality standards, aquatic organisms, and human Acceptable Daily Intake levels were very large.