This paper outlines the application and usefulness of a software platform that enables hydrologists to develop custom functionality in a new hydrological modelling tool, eWater Source, designed for water resources planning and management. The flexible architecture of the software allows incorporation of third-party components as plug-ins to add new capabilities that are not built in. Plug-ins can be developed to adapt the software to suit the needs of hydrologists with modest software development knowledge. This can result in an improvement in workflow and efficiencies. In addition, modellers can use plug-ins to integrate hydrological process and management models that may not be able to be built in the normal tool. The paper introduces the plug-ins functionality of the modelling tool, its design and applications with three example plug-ins to demonstrate. These are: (1) a data processing plug-in to upscale urban environment models; (2) a management rule plug-in to calculate loss allowances for the Pioneer Valley; and (3) a model plug-in to integrate into a river system model. For planning purposes, the use of plug-ins is thought to be critical for modelling management rules for various jurisdictions since these can vary significantly between jurisdictions and change over time.