Pb, Sr, and Nd isotope data for Cenozoic basalts in eastern Australia indicate that mantle isotope signatures of Pacific Ocean mid-oceanic ridge basalts (MORB) type characterize the lava-field basalts (55–14 Ma) in southeastern Australia, whereas mantle isotope signatures of Indian Ocean MORB type characterize younger basalts (6–0 Ma) from northeastern Australia. This discovery further constrains the secular distribution of major asthenospheric mantle reservoirs represented by the Pacific and Indian Oceans MORB sources during and following the breakup of eastern Gondwana and tracks, for the first time, the locus of the boundary of the two reservoirs beneath the Australian continent. These new data fill the gap between previous Indian Ocean MORB–Pacific Ocean MORB boundary locations determined from backarc basin basalts in the southwestern Pacific Ocean and ocean-floor basalts in the Southern Ocean. We propose that the Indian Ocean MORB source is a long-term asthenospheric reservoir beneath most of eastern Gondwana and that the westward migration of the Pacific Ocean MORB source may have been associated with the opening of the Tasman Sea (ca. 85–60 Ma) along a broad front southeast of the Australian continent.