"You might expect cliffs to be more permanent, but those at Tongaporutu are not. They have been fashioned from crumbling grey papa and sandstone, and are under constant assault from the volatile Tasman Sea. Anecdotal evidence suggests that the coastline is eroding at the rate of two metres annually. Changes are thus occurring over breathtakingly short timeframes even in human terms, let alone the normal spans of geology.
Spooky caves, stoic rock stacks and great sheer faces of layered rock—none of them last."
....for more read on at nzgeographic.co.nz/tongaporutu-coastline