Last weekend's high seas also undermined the road beside the beach and swallowed about 8m off the Punakaiki Beach Camp.
Camp lessee Craig Findlay, who also chairs Punakaiki's special rating district for the seawall, said all properties on the beach front were at risk.
"It was just a combination of high tide and high seas coming in from the southwest. The problem was exacerbated by the fact it wrapped around the corner of the existing seawall and created a cut-out."
Mr Findlay said the sea had now pushed into the trees protecting the camp from the prevailing westerly winds.
"We're really in danger of losing a lot of the amenity value of the camp should it go any further."
Mr Findlay and his wife Sue have a 20-year lease of the camp from the Buller District Council and are in their third summer there.
He said it was the worst erosion he had seen in such a short period.
"Absolutely. This was like two tides. It was a matter of 48 hours."
He said the West Coast Regional Council was proposing emergency work which would be charged back to ratepayers.
Potentially, the northern end of the seawall was also at risk.
Another Punakaiki resident, Richard Arlidge, said he had come home after three weeks away to find the sea had eaten a "huge chunk" of the foreshore.
"At the moment the road's really dangerous. The vertical drop ... is right on the edge of the road."
Regional council planning and environment manager Mike Meehan said he believed the sea had simply come over the top of the seawall.
He said the council was seeking prices from contractors for repair work.
It was also inspecting the northern end of the foreshore today to see how much more had eroded.
"We think there was something like 8 to 10m."
Mr Meehan said the council had been working since last year on a proposal to protect the northern end, and putting together a report to consult the community. The options included extending the seawall or less expensive proposals such as building up the beach
The seawall remained intact but needed more rock at the northern end. Some of the fabric beneath the rocks had also come away and would need replacing.
It was the worst erosion in a short time he had seen at Punakaiki.
"The Punakaiki Rating District have been hammered with contracting bills over the past four or five years, just due to the lack of sediment coming through and filling up that beach and constant pressure on the sea wall from the sea."
The wall was under much more pressure, especially from king tides, than when it went in five years ago, he said.
The regional council was also seeking advice from consultants on whether the erosion was likely to worsen.
- Westport News