The primary objective of this study was to perform a detailed engineering geological investigation of the proposed Island Block Opencast Coalmine highwall at Reefton. An investigation was undertaken to provide information on the rock material and rock mass properties of the Brunner Coal Measures that comprise the highwall, with the specific aims of identifying the distribution of rock types and the locations and orientations of mappable defects such as faults, joints, shears, and crush zones. The Brunner Coal Measures that comprise the overburden are an alternating sedimentary sequence of massive sandstones, laminated sandstones, siltstones, mudstones, and coal. These were divided into 5 geotechnical units on the basis of physical, and mechanical intact rock material parameters determined for each. The coarse-medium-grained lithologies typically have high porosities (n=8.1-10.6%), and friction angles (41.5 and 47.1°), and low dry densities (2290-2390kg/m\ slake-durability index results (LI2=65.0- 87.4% retained), UCS intact rock strength (21.2-34.3MPa), and cohesion (5.7 and 8.8MPa). The laminated fine-grained lithologies have low porosities (n=2.0-4.8%), and friction angles (34.8-43°), and high dry densities (2480-2570kg/m3 ), slake-durability index results (LI2=98.5-98.8% retained), UCS intact rock strength (36.7-60.8MPa), and cohesion ( 14.3-15.8MPa). Scanline survey traverses were conducted along the existing highwall benches to characterise the rock mass at Island Block, and the rock mass properties determined were; defect type, chainage, dip and dip direction, persistence, aperture, nature of infilling, defect roughness, amplitude and waviness, and ground water flow. Joints are typically steeply dipping, with mean joint set orientations on the eastern highwall for 11 of 70-901236°, J2 47-59/240°, and 13 where present 65-811261°. Normal faults present in the eastern highwall have orientations similar to the joint set orientations. The joint sets in the southern highwall display more variance in their orientation: 11 - 80-901210-232°, 85-90/360° or 85- 901180°, 821278°, and J2 - 80-901122°, 80-901295°, 821235° and 891099°. Faults in the southern highwalls typically display a strong E-SE dip direction, with orientations ranging between 054-137° and dips 30°_70°. Structural domains were developed for the high walls around Island Block based on the joint orientations, and the presence of faults. These were further classified on the potential mode of failure after kinematic stability analysis was performed on the joints sets, faults, and the combination of these two defects. Potential toppling failure on joints was found to be the dominant failure mode, although this mode of failure was observed in the field to lack the upper and lateral release surfaces, necessary for failure. The structural history of the Brunner Coal Measures at Island Block has been characterised by intense periods of compression-and extension-related deformation, due to structural deformation at a region level. Thrust faults and imbricate thrust duplexes have developed in response to compression from the north. Joints are both systematic and non-systematic as result of the differing styles of deformation. Normal faults recorded on the eastern highwall formed either coeval with the thrusting or after valley incision. Wedge shaped seam splits are present in the western section of the southern highwall, and are observed to pierce the 'A' and B' coal seams. The development of these splits related to syndepositional faulting. The mining implications of these structures relate to the overall stability of the highwalls, as they provide planes of weakness within the rock mass along which failure can occur.