Knowledge of the in situ rock stress is much needed to predict response of the rock mass and safely implement the underground structures. The reliable estimation of in situ rock stress is always a challenge due to its complex nature as it is influenced by various factors such as the weight of overlying rock mass, topography, geological structures, tectonic forces, and residual/remnant stresses. The estimation of in situ rock stress using methods like overburden criteria and thumb rules have some limitations and hence difficult to completely rely on. Especially for unlined pressure tunnels for hydropower, reliable estimation of in situ stress state is much crucial since these tunnels are subjected to the direct hydrostatic water pressure. If the in situ minor principal stress magnitude is lower than the induced hydrostatic water pressure, hydraulic failure may take place which may lead to water leakage out of the tunnel. The aim of this article is to develop a 3D numerical rock stress model covering large area of the topography which consists different large-scale discontinuities such as regional faults and weakness zones. To do so, the authors first establish reliable input parameters and boundary conditions. Then, the in situ stress state of the surrounding area and along an unlined high pressure headrace tunnel of the Røldal hydroelectric project will be assessed. Finally, the predicted results of in situ rock stress from the 3D numerical modeling will be compared with the in situ measured rock stress. It will be demonstrated that it is possible to create a large-scale 3D numerical model, which provides overall distribution of in situ rock stress in an area with complex geological settings consisting of faults and weakness zones.