Collaborative governance is frequently thought to improve legitimacy and environmental outcomes. However, there remain gaps in our knowledge of whether and how collaborative governance influences legitimacy. Drawing on political systems theory and using a comparative case survey method, we explore legitimacy dimensions at the input, throughput, output, and outcome stages in 14 collaborative decision-making process in Aotearoa New Zealand from 2009-2017. Our qualitative inquiry utilizes coding categories that intertwine legitimacy with collaborative governance indicators. The analysis of these freshwater cases highlights the importance of input legitimacy within and across cases, emphasizing the critical nature of defining roles, responsibilities, authority, and mandate before collaboration. We delve into the implications of these findings for collaborative efforts in Aotearoa New Zealand and on an international scale.