This paper focuses on migration between Australia and New Zealand, which has exhibited a strong, but cyclical, net movement towards Australia since the late 1960s. A long-term historical perspective is taken. Trans-Tasman migration is also compared with inter-island migration within New Zealand. It is argued that differential economic development, driven by forces of globalization, agglomeration and technological change, has been primarily responsible for the long-run changes in the distribution of population across the regions of Australasia. Asynchronous business cycles, demographic dynamics, perceptions, return migration and the high international mobility of New Zealanders (of whom one quarter of those aged 40-64 have lived abroad for a year or longer) are responsible for the short-run fluctuations. However, permanent and long-term migration is only a small fraction of total trans-Tasman population movement. Moreover, trans-Tasman migration has not offset New Zealand's ability to recruit population through immigration. Over the last three decades, the outflow of more than half a million New Zealand citizens has been compensated by a net inflow of three-quarter million citizens from elsewhere. The number of New Zealanders in Australia is expected to continue to grow but the migration flows are becoming increasingly diversified. One-third of the New Zealanders in Australia remigrates within four years. Future trends will depend on New Zealand's ability to boost productivity growth, the real cost of air travel, retirement migration and the impacts of climate change.