The prevalence of block and panel caving mines is predicted to increase as easy access deposits exploited with open pit mining are worked out and mining of deeper deposits is required to meet metal demand. With the caving process from block and panel caving mines relying strongly upon the natural fracture network of the rock mass, there is an associated need for improving the characterization of the in situ fragmentation. In-situ fragmentation is defined as the blocks that are naturally present in the rock mass before any mining activity occurs. In the various forms of block and panel caving, the in-situ fragmentation of the rock mass strongly influences cave initiation, cave propagation, and the draw of ore. Increasingly discrete modelling methods are being used to address the challenges of characterising the in-situ fragmentation. However many of these approaches are constrained by the need to simplify the fracture system down by using some rather unrealistic assumptions. For instance the use of infinite ubiquitous fracturing are clearly properties not observed widely in nature but commonly implemented. The recent increase in the common application of borehole imaging tools and advanced photogrammetry to accurately describe the orientation, extent and geotechnical character of the discontinuities in a rock mass now provide the data to do better. With our increasing ability to image and quantify the rock mass structure, more accurate models of the actual rock mass are available for rock engineering design purposes. The use of realistic fracture network models offers a number of advantages over existing methods: the probability of complete block formation can be determined rather than just identifying the possibility. The detailed geometry of identified blocks can be defined in terms of mass, volume and shape as well as the associated rock bridges that are preventing full block formation.