Forty-two rock avalanche deposits, with volumes ranging from a chosen lower threshold of 1 × 106 m3 up to 500 × 106 m3, have been identified in a 10,000 km2 area of the central Southern Alps, New Zealand. The observed frequency of one per 244 yr over 10,250 yr is biased by erosion. A nearly complete sample of the past 1,700 yr indicates a frequency of 1 per 94 yr. On the average, the largest rock avalanche in any century is 56 × 106 m3, and in any millennium, 103 × 106 m3. At about one per century, most large rock avalanches are probably triggered by large earthquakes and only a few by large storms. The risk of rock avalanche debris reaching a specified distance from a source of known elevation is given by a simple stochastic model, locally calibrated to assess rock avalanche hazard.