Volcanic domes may be emplaced rapidly and with few hazardous consequences, even at the summit of large stratovolcanoes. In this study the most recent activity of Mt. Taranaki in New Zealand is shown to have been a passive effusion of a c. 5.9millionm3 lava dome with minor associated explosions and little syn-eruptive hazard. This event, the Sisters eruption, appears to have been unrecorded by local indigenous populations but likely occurred between A.D. 1785 and 1820. The magma erupted is chemically distinct from the preceding A.D. 1755 Tahurangi eruption. Based on breakdown of hornblende crystal rims, the Sisters magma was probably only four days outside the hornblende stability field before cooling, and the magma ascended its last four km along a conduit at rates of 0.012ms-1. Based on dome surface morphology, a relatively low-viscosity magma is inferred. The dome remained in a metastable state for up to 70years following the eruption; eventually generating a large, cool (<350°C) collapse of at least 2millionm3 of rock, forming a highly hazardous mass flow that travelled over 5km. Factors contributing to the post-eruption dome instability include: its emplacement onto a steep flank of unconsolidated breccia or talus, possible oversteepening, fracturing due to spreading and subsidence, rapid cooling during heavy rain, and a hydrothermally altered inner core. The likely trigger for the collapse was a heavy rainstorm or an earthquake. Volcanic domes in the summit regions of volcanoes must be considered metastable and a potential source for hazardous, non-eruption related mass flows for many decades following their emplacement.