<p>One of the viable solutions that can help to achieve cost effectiveness and environmental sustainability in the hydropower project is the use of unlined pressure tunnels and shafts as the waterway system. The concept behind an unlined pressure tunnel and shaft is based on the principle that the inherent rock mass directly resists the pressure exerted by the hydrostatic water head. This manuscript presents a unique case scenario of an unlined pressure tunnel system of the Nye Tyin Hydropower Plant in Norway, which is an ultra-high head unlined pressure tunnel that upholds the maximum hydrostatic water head of approximately 1047&#xa0;m. The manuscript assesses the in-situ stress state along the unlined pressure tunnel of Nye Tyin hydropower project by creating a 3D numerical model with the integration of topography, actual geological and local geotectonic conditions focusing mainly on minimum principal stress state. The spatial distribution of minimum principal stress along the unlined pressure tunnel is investigated and quantified using numerical modeling. The field measured in-situ stress data along with rock mechanical data from the project area are used to validate the in-situ stress state from the numerical model. In addition, a large-scale model was developed to evaluate on the far-field stress magnitude influenced by a regional fault that crosses the project area. The manuscript shows that it is feasible to thoroughly assess the in-situ stress state throughout an entire waterway system using a 3D numerical model combined with field measured in-situ stress data. The assessment of in-situ stress perturbations at the tunnel-fault intersection is carried out using the numerical model and compared with the Norwegian thumb rule. Furthermore, a sensitivity analysis of rock mass strength parameters in relation to the fault zone stiffness parameters is performed, providing new insights into their influence on minimum principal stress. </p>