Soil structure affects the movement of water in hillslope soils and therefore exerts a strong influence on slope stability. A debris flow is analysed within the confines of an instrumented catchment on the South Island, New Zealand, in order to examine the influence of soil macropores on slope stability. Tensiometric and slope throughflow data for nearby slope areas show that vertical cracks conduct rainfall at rates well in excess of the mineral soil matrix conductivity. The presence of a well connected pipe system at the soil-bedrock interface distributes this water quickly downslope. Under exceptionally high rainfall intensities, however, this crack-pipe system may induce slope instability by increasing the rate of infiltration over lateral pipeflow rates. This results in a build-up of pore pressure at the soilI–rock interface and subsequent slope failure.