Slow-moving landslides are a major hazard but their movement mechanisms during earthquakes and rainstorms are not fully understood. We collected samples from a slow-moving landslide complex in New Zealand and subjected them to a range of porewater pressure and dynamic stress scenarios in a dynamic back-pressured shear box. Our results show how the complex movement patterns, observed in many large slow-moving landslides, may be mobilized by strong earthquakes and significant rain events.