Hydraulic simulation models have become a valuable tool to manage water distribution networks commencing from their initial design through their operation, assessment of the level of service to customers, system performance improvement, analysis of planning alternatives, to system optimisation. The development of hydraulic models can be a time consuming task with complex and large scale water distribution networks being particularly challenging. The Wimmera Mallee Pipeline (WMP) in Western Victoria, Australia is a recently constructed and unique regional scale water distribution system which consists of over 8,800 km of pressurised pipelines spreading across an area of approximately 20,000 km 2. Currently, the WMP provides water to 34 townships, rural farms and other water users across the Wimmera Mallee region with an annual design capacity of 31.6 mil. m 3. The WMP sources its water from multiple reservoirs in the Grampians mountain ranges in the south and the River Murray in the north. Grampians Wimmera Mallee Water (GWMWater) is the local water organisation responsible for managing the WMP. GWMWater is currently initiating the development of a water market to generate and support growth, and to ensure that water is available for the highest value social, economic or environmental use. The hydraulic models discussed in this paper will assure that informed decisions are made by GWMWater regarding the capacity to deliver water through the pipeline system, and therefore the extent of trade by customers. The philosophy for development of the WMP hydraulic models was to replicate the real system as credibly as possible into the level of required accuracy for decision making, yet enable simple model operation, maintenance and update. The network is modelled to the individual customer level in order to accommodate small diameter pipes. Modelling at this level simplifies the future model maintenance and updates, and also ensures the compatibility with other GWMWater's systems such as Geographic Information System (GIS) and the customer database. A major part of the model development consisted of data preparation. This was undertaken by using "as constructed" GIS asset data captured during the WMP construction and entered into a GIS database (ArcGIS by ESRI). Due to the scale of the system and associated amounts of data, it was essential to develop sophisticated data transformation and validation procedures to simplify the model build which thereby minimised manual data entry and potential sources of errors. This paper focuses on the methodologies and techniques used in data preparation for hydraulic models and development of hydraulic models. An example is also provided of how the models will be used as a decision support tool in water supply and allocation planning.