This study investigates the influence of humidity on the high voltage behaviour of zinc oxide porous pellets at room temperature, using the Phase Resolved Partial Discharge (PRPD) method. The experimental configuration corresponds to the one that would be used for flash sintering cylindrical ZnO pellets at low temperatures in possibly scalable conditions: planar Pt electrodes without Pt paste. The study explores the incubation period by increasing the 50 Hz AC electric field up to 8 kV/cm (applied by steps or with a continuous ramp) in an air atmosphere with varying relative humidity. Experiments were repeated on several samples and showed that the maximum amplitude of partial discharges decreased while their number increased as the relative humidity increased. With the support of video recording of the experiments, different characteristics in the PD pattern evolution with increasing AC voltage could be attributed to various locations between the electrodes, at the sample surface, or at the interface with the electrodes. By examining the PRPDs, it was possible to identify two distinct types of partial discharges having different thresholds of inception: one type corresponds to partial discharges occurring mostly at the triple junction “ZnO-platinum-air” or in the ZnO-electrodes gap and depends strongly on the relative humidity level and; the other type corresponds to electric arcs between the electrodes, with an inception threshold much less affected by the relative humidity level.