Over the past 150 years Great Barrier Reef (GBR) catchments have been extensively modified for agricultural production leading to a decline in water quality entering the Great Barrier Reef lagoon. In response to these water quality concerns the Reef Water Quality Protection Plan (Reef Plan) 2003 was initiated and updated in 2009 through a joint Queensland and Australian government initiative. The Reef Plan outlines a clear set of water quality and management practice targets for sediment, nutrients and pesticides. Improvement in water quality is achieved through government investment into improved agricultural management practices. A Paddock to Reef Integrated Monitoring and Modelling and Reporting program has been established to measure and report progress towards meeting reef targets, and to assess the benefits of improved land management practices on water quality discharged to the reef lagoon. The eWater CRC Source Catchment modelling framework and an ensemble of paddock models are used to model key agricultural industries and constituent loads from 35 major GBR catchments, with a combined area of 424,000 km2. In addition modelling outputs have been used in a multi-criteria analysis to assist policy-makers in prioritising future funding investments. This session will provide an insight into a highly collaborative and innovative modelling and monitoring program extending from paddock scale through to the receiving waters.