Decadal and long-term variability of sea level in the Southwestern Pacific (SWP) was investigated by using tide gauge measurements obtained near New Zealand during 1948–2018 and a 1.5-layer reduced gravity model. After the removal of the global mean sea level, there still exists a prominent positive trend and strong decadal oscillations in SWP sea level, which are 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 SWP sea level rise (SLR) through Ekman convergence. On the decadal scale, locally generated and westward propagating signals induced by IPO exert comparable influences on sea level variations in the SWP. Results from multivariable linear regression suggest that SAM can account for almost all the long-term SLR, and IPO explains approximately 58% of the decadal variability.