An intermediate-composition hydroclastic breccia deposit is exposed in the upper reaches of a deep glacial valley at Ruapehu volcano, New Zealand, indicating an ancient accumulation of water existed near the current summit area. Lobate intrusions within the deposit have variably fluidal and brecciated margins, and are inferred to have been intruded while the deposit was wet and unconsolidated. The tectonic setting, elevation of Ruapehu, and glacial evidence suggest that the deposit-forming eruption took place in meltwater produced from an ancient glacier. The breccia-lobe complex is inferred to have been emplaced at > 154 ± 12 ka, during the penultimate glacial period (190–130 ka) when Ruapehu’s glaciers were more extensive than today. This age is based on overlying radiometrically dated lava flows, and by correlation with a well-constrained geochemical stratigraphy for Ruapehu. Field relations indicate that the glacier was at least 150 m thick, and ubiquitous quench textures and jigsaw-fit fracturing suggest that the clastic deposit was formed from non-explosive fragmentation of lava in standing water. Such features are unusual for the high flanks of a volcanic edifice where steep topography typically hinders accumulation of water or thick ice, and hence the formation and retention of hydroclastic material. Although not well-constrained for this time, the vent configuration at Ruapehu is inferred to have contributed to an irregular edifice morphology, allowing thick ice to locally accumulate and meltwater to be trapped.