Sudden releases of impounded water from lakes in volcanic regions constitute a major and frequently repeated hazard. An outburst flood from Taupo caldera, New Zealand, released ∼20 km3 of water, within decades following an ignimbrite-emplacing eruption, ca. 1.8 ka. Paleohydrologic reconstruction of the Taupo flood provides estimates of peak discharge at the outlet in the range 17 000–35 000 m3/s. Dimensionless analysis demonstrates that (1) failure of the barrier was essentially instantaneous, (2) the event may be treated hydraulically as a dam break, and (3) the peak discharge was a function of outlet geometry rather than lake volume or breach formation rate. Paleohydraulic reconstructions based on empirical relations derived from historic dam breaches yield only order of magnitude estimates of peak discharge. Calculations based on the physical dimensions of the outlet channel and hydraulic principles are likely to be more accurate and are in close agreement with computer-implemented dynamic-flow–routing models. The latter give peak discharges and maximum stage levels similar to constraints imposed by field evidence and estimates of flow depth and velocity. The long duration of the Taupo flood and the relatively narrow, confined flood route resulted in minimal attenuation of the flood wave compared with modern dam breach events, and flood deposits can be traced as far as 232 km downstream. Caldera lake breakout floods may be among the most far-reaching hazards associated with volcanism.