In February 2023, Cyclone Gabrielle caused intense rainfall resulting in severe soil erosion, flooding and sedimentation on the coastal plain and offshore on a scale not witnessed since Cyclone Bola (1988). Large-scale and widespread impacts on the environment, communities and infrastructure occurred in Te Tairāwhiti/Gisborne and Te Matau-a-Māui/Hawkes Bay regions of the North Island, underpinning New Zealand’s biggest weather disaster. Compound specific stable isotope (CSSI) sediment source tracing was used to determine the relative contributions of major rivers along the coastline. Seabed samples from six cross-shelf transects also determined the balance of terrestrial versus marine influences. Analyses confirmed the recent terrigenous origin of these mud-rich deposits and could distinguish specific riverine sources. This finding contrasts previous CSSI studies at other New Zealand coastal settings that showed marine sediment typically make a substantial contribution to estuarine and marine sedimentation. The overwhelming terrigenous character of the cyclones’ sediment deposits reflect the sensitivity and vulnerability of the East Coast's mountainous and unstable landscape to extreme rainfall that triggers massive soil erosion and event sedimentation along the coast. This work can be used to understand land-based stressor impacts and will ultimately provide information to support decisions about future land management on the East Coast.