New Zealand sits on the boundary between the Indo-Australian and Pacific tectonic plates. In the South Island, most of the relative movement between these plates concentrates along the Alpine Fault, a major strike-slip fault with a significant reverse component that runs the length of the island's western edge. The 1968 Inangahua earthquake occurred along the fault's northern section and was, in geological terms, considered average for what the Alpine Fault can produce. Fifteen aftershocks of magnitude 5 or greater followed within a month, each one a reminder to residents that the ground beneath them had not finished settling. The surface-wave magnitude reached 7.4, and the shaking was felt across much of the South Island. More than two-thirds of chimneys in Greymouth, Westport, and Reefton were damaged. Near Greymouth, a man died when his car hit a section of road that had subsided on the approach to a bridge.

The earthquake and its aftershocks triggered landslides across the surrounding mountains, but one was catastrophic. A massive slide dammed the Buller River upstream of Inangahua, and the rising water backed up for seven kilometres, lifting the river 30 metres above its normal level. If the dam had burst, the flood would have swept through not only Inangahua but the much larger town of Westport downstream at the river mouth. Authorities evacuated roughly 10,000 people as a precaution. In the end, the river overflowed the debris dam and gradually eroded through it, avoiding the worst-case scenario. Meanwhile, within the 50-kilometre stretch of Buller Gorge, the road was blocked in more than 50 places by landslides or where the road surface itself had collapsed into the gorge. More than 50 bridges were damaged or destroyed. Two goods trains were derailed, and over 100 kilometres of railway track had to be replaced.

In the hours after the quake, the people of Inangahua gathered at a relief centre in the Ministry of Works yard and made a disturbing discovery: radio broadcasts were reporting only minor earthquake activity. The rest of the country seemed unaware of the scale of what had happened. Inangahua remained isolated until a truck driver managed to reach Gisborne on his radio. By noon, RNZAF Bell UH-1H helicopters and commercial aircraft were arriving with aid and surveying the damage. A constable and a doctor walked to Inangahua Junction from Reefton, carrying medical supplies on their backs. The doctor treated patients outdoors because aftershocks made buildings unsafe. A group of about 50 people set out on foot from Inangahua toward Reefton, a walk of roughly seven hours through landslide-scarred terrain. Helicopters ferried others to Rotokohu, where buses waited to take them onward. In all, 235 people were airlifted out.

Inangahua never fully recovered. Many residents who evacuated chose not to return, settling instead in Reefton or leaving the region altogether. The fear of aftershocks was a powerful motivator, but so was the sheer scale of destruction. Damaged buildings took months or years to restore. The Inangahua School was destroyed, and children transferred to Reefton School, uprooted from their routines and their friends. Employment fell, school rolls shrank, shops lost customers, and Help Wanted signs proliferated in a town with fewer and fewer people to read them. The railway, at least, was fully restored, as it formed a key part of the Stillwater-Westport Line. But the human infrastructure proved harder to rebuild. Families who had lost everything received supplies from the Red Cross and other agencies, and some emerged materially better off than before. But the community itself, the dense web of relationships and routines that made Inangahua a town rather than a collection of buildings, had been shaken apart. The economic decline that followed was the slower, quieter disaster that outlasted the tremors.