Acquiring accurate eruption records and understanding the volcanic processes behind eruption periodicity are important in the development of realistic hazard assessments and volcanic emergency planning. Here, we use a detailed study of the Holocene (<10,000 yr B.P.) record of Mount Taranaki (New Zealand) to explore the link between magmatic processes and eruption frequency, helping to demystify these outwardly complex andesitic magma systems. Six compositionally distinct magma batches were identified throughout the Holocene record at this volcano, many of which show similar evolutionary paths. The batches erupted on 1500-2000 yr time scales, which were synchronous with variations in eruptive frequency. We suggest that a progressively developing lower-crustal hot zone is the source of these magmatic batches. The largest-volume (>0.5 km3) eruptions appear to be statistically predictable because they tend to occur just prior to a period of repose, and they erupt the most strongly evolved magmas. The fundamental properties of magma-volcanosystems identified here offer a paradigm for constraining the time scales and nature of magmatic processes, in addition to providing a foundation for more robust probabilistic time-varying hazard forecasts.