This study investigates the influence of the El Niño–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO) on rainfall and streamflow regimes of eastern Australia. An analysis of historical rainfall and streamflow data for Queensland (QLD), New South Wales (NSW), and Victoria (VIC) reveals strong relationships between these indices and seasonal rainfall and streamflow totals. Rainfall and streamflow in NSW and QLD are shown to be significantly enhanced during the La Niña phase of ENSO, with La Niña impacts diminishing as one moves south into VIC. In addition, the study shows that on a multidecadal timescale the negative phase of the IPO is associated with “wetter” conditions than the positive phase. Importantly, the already enhanced La Niña rainfall and streamflow is demonstrated to be even further magnified during La Niña events that occur in the IPO negative phase. This result is of particular importance as the influence of ENSO in VIC appears to be weak; however, the results indicate that some useful predictability of ENSO impacts can be achieved during the negative phase of the IPO for VIC.