This paper details the development and design of the Observatorio Metropolitano de Agua para Lima-Callao (the metropolitan water observatory for Lima-Callao, MWO). The MWO is a digital, collaboratively developed observatory that aims to collect and share data about water access and infrastructuring practices within the metropolitan city of Lima-Callao, Peru. The purpose of developing the MWO has been to contribute to a fairer distribution of water resources amongst urban residents by creating an ‘espacio de concertación’ and collect and diffuse data on access to and quantity and quality of water for human consumption. By combining collaborative design approaches with the theory-informed data justice principles, we have been able to develop a prototype of the MWO. In general, this teaches us how to design digital platforms according to the principles of data justice in practice.