The local deterioration of the mechanical properties of steel-lined pressure shafts and tunnels induces a decrease of the celerity and an increase of the energy attenuation of water hammer waves. This deterioration may arise from the weakening of the backfill concrete and the surrounding rock mass, which provide the radial support of the steel liner. A new online monitoring system implemented and tested at the Grimsel II pumped-storage shaft in Switzerland is presented. The difficulties encountered during the in situ measurements are outlined. The new monitoring approach is based on acquiring and analyzing the dynamic pressure data generated by the normal operation of pumps and turbines. The wave celerity and the exponential attenuation coefficient of water hammer waves are estimated based on pressure records. Monitoring charts for these indicators are established using the statistical quality control method. The results show a stable monitoring scheme that needs longer acquisition data series to consolidate its control limits.