Freshwater is central to New Zealand’s wellbeing, supporting the economy, recreation and Māori customs, knowledge and food gathering. Changes in land use over the past three decades have degraded water quality, but the details are more complex than the public discourse. Concern about water quality degradation led to the ‘limit setting’ approach and the National Policy Statement for Freshwater Management (NPS-FM). From its inception in 2014, the key focus of the NPS-FM has been on the management of diffuse discharge of contaminants from land. Limit setting starts by defining environmental objectives and then devises rules for land use that will restrict contaminant discharges so that objectives can be achieved. The aims are laudable and are conceptually similar to the Paris Climate Agreement that set the environmental objective of a less than 1.5oC temperature rise and then aims to limit greenhouse gas emissions to achieve this. However, implementation of the approach is dependent on modelling and there are numerous technical challenges including defining appropriate criteria, attributing effects to causes, using models and simulations to devise defensible resource use limits, and appropriately accounting for and communicating uncertainty. The challenges are multi-disciplinary in nature and require strong leadership to address. There is significant responsibility and pressure on lead modellers because modelling is always a compromise between reliability, usefulness and feasibility. Pressure is most evident in the final step of the implementation process where models and model outputs must be defended in intimidating and adversarial formal decision-making process such as the Environment Court. Some of these pressures could be reduced if adjustments were made to the NPS-FM to acknowledge the technical challenges. Notwithstanding the challenges, implementation of the NPS-FM can be a fulfilling career for people with the right skills and qualities.