Wind turbine lightning stroke diagnostic apparatus for a wind turbine having conductive blades. A lightning protection system input is provided for receiving measured current data associated with a current conducted by the lightning protection system following a lightning stroke. An air pressure sensor is mounted using a mount within an internal cavity of a conducive wind turbine blade assembly. A sensor monitor monitors the measured current data and the output of the air pressure sensor for identifying a bypass lightning stroke when a measured current increase coincides with the detection of a lightning generated shockwave within the internal cavity by the air pressure sensor.