The eastern bent-wing bat (Miniopterus orianae oceanensis) is a small (11–20 g, mean 14 g) insectivorous bat with a distribution that extends along the eastern seaboard of mainland Australia. It is primarily a cave-dwelling species, particularly for breeding females who form large maternity colonies at just a few locations throughout its range. Seasonal population changes at one of the three large New South Wales maternity colonies (Church Cave) were studied from December to March every year for 12 years when adult females were resident at a maternity site. Five key periods were identified: (1) adult arrival, (2) adult peak, (3) juvenile independence, (4) adult–juvenile peak, and (5) autumn migration. The average duration of the adult peak period was 38 days and usually commenced around late December or early January. This is the critical period in which to estimate the female adult population. All other periods lasted ~14 days. Understanding the timing of these different periods is important in estimating various population parameters. The timing of migration is also important with respect to windfarm construction and impact assessments of turbine strike to migrating bats. Four separate variables were investigated to describe the timing of autumn migration from Church Cave; moon illumination, minimum nightly temperatures, barometric pressure, and timing of adult arrival. The timing of adult arrival was the only model that was significant in explaining the onset of migration. This generally occurs 83–87 nights after the commencement of arrival of female adult bent-wing bats at Church Cave in early to mid December.