GNSS Tomography is an all-weather remote sensing method to retrieve the tropospheric water vapour due to the continuous pass of GNSS signals through this part of the atmosphere. This method allows continued monitoring of the water vapour and other related parameters at a low cost and reasonable spatial resolution compared to conventional measurement techniques, like radiosonde and radiometer. To achieve this goal, the troposphere is discretized into a number of 3D elements (voxels) in the horizontal and vertical directions. Then, using GNSS signals which cross voxels, the spatial and temporal structure of the wet refractivity in this part of the atmosphere is reconstructed. This research investigates the performance of the GNSS tomography technique in the mountainous region of Austria. Test computations are based on the data from the EPOSA (Echtzeit Positionierung Austria) GNSS network, which is mainly covered the western part of Austria. The span DoYs 187-201 in July 2021 has been chosen as an experimental period due to the high amount of precipitation leading to flooding during that time. Finally, radiosonde observations gained at Innsbruck airport (RS11120) have been utilized as a reference to analyze the accuracy of the reconstructed wet refractivity profiles.