An Earth tide signal at the lunar tidal period of 12 hours 25 min has been detected in the Sagnac frequency record of the C-II ring laser 30 m underground at Christchurch, New Zealand. Its amplitude, one part per million of the Earth rotation signal, is much greater than the value of 40 parts per billion expected. Tiltmeter records show that a substantial part of this amplification is geophysical, the lunar component of tilt having an amplitude of the order of 0.1--0.2 ¿rad, principally because of ocean loading of Banks Peninsula. The joint records also show effects on the Sagnac frequency associated with cavern deformation under ambient pressure and temperature change and with long-period waves in cavern tilt.