Decadal and long-term variability of sea level in the Southwestern Pacific (SWP) is investigated with tide gauge measurements near New Zealand ranging from 1948 to 2018 and a 1.5-layer reduced gravity model. After removing the global mean sea level, SWP sea level still exhibits a prominent increasing trend accompanied by strong decadal oscillations, which is attributed to the Southern Annular Mode (SAM) and Interdecadal Pacific Oscillation (IPO), respectively. On the long-term time scale, the intensification of local wind stress curl associated with strengthening SAM contributes to the SWP sea level rise through Ekman convergence. On the decadal time scale, locally generated and westward propagating signals induced by IPO exert comparable influences on the sea level variations in the SWP. Multi-variable linear regression analysis suggests that SAM and IPO account for approximately 83% and 58% of the tide gauge sea level variability on long-term and decadal time scales, respectively.