This paper uses responses from a regional farmer survey that identify farmers’ perceptions of environmental policies to calibrate a catchment-level environmental economic model (NZ-FARM) to estimate the impacts of a nutrient reduction policy in North Canterbury, New Zealand. The model maximizes farm income across a catchment, accounting for changes in land use, farm output, nutrient leaching, and GHG emissions. Simulations estimate that reducing nutrient loads by 15–30% can be achieved with economic impacts ranging between 1 and 10%, based on how willing landowners are to change how they manage their farm. Farmers are often hesitant to implement certain mitigation options, however, which results in higher economic costs than the ‘optimal’ estimates. Farm-level impacts will likely vary through the current farm practice, the farmers’ attitude towards the regulation, and the ability of policymakers to educate and incentivise landowners to adopt a variety of land management options.