Holocene paleoseismic activity on the Akatore Fault zone, southeastern New Zealand, has caused uplift of a 23 km section of coastline by several metres. Prominent relict shoreline terraces are preserved at 6 m and 3 m above the present sea level, and the latter terrace was formed 1000-1400 yrs BP. The main fault strand farther inland has 6 m of late Holocene vertical offset, but the relationships between coastal offsets and fault offsets are not understood. There is no preserved geological evidence on the coastline to distinguish between incremental uplift (e.g., numerous centimetre-scale events) and major, metre-scale, uplift events: a distinction that is important for evaluating regional paleoseismicity. We have used genetic characterisation of populations of live kelp, Durvillaea antarctica growing along the shoreline to investigate whether or not there has been a catastrophic uplift event, greater than the two metre tidal range, that was sufficient to extirpate intertidal kelp populations. Our results show that all kelp along the southeastern New Zealand coastline belongs to the same distinctive clade of D. antarctica, and inhabits the same ecological niches on exposed rocks and reefs. However, the kelp population on the uplifted coastline is genetically uniform, and clearly distinct from those of flanking shorelines. This regional genetic anomaly is consistent with a major extirpation event, followed by recolonisation from source populations some 30-120 km to the northeast of the uplifted coast. The uplift event caused vertical crustal movement with similar amounts of uplift on the fault plane and the coastline 3 km away. This approach, using genetics of intertidal biota, has potential applications for evaluation of paleoseismicity of other tectonically active shorelines around the world.