Tens of thousands of plastic bags are littering a West Coast beach after high seas ripped open an old coastal rubbish dump.

Huge damage from ex-Cyclone Fehi's rough seas and king tides ripped away the beach at Cobden Tip Head and Shipwreck Pt, Greymouth.

"It is devastating," Grey District Mayor Tony Kokshoorn said.

"Tens of thousands of plastic bags are all across the beach. I have never seen anything like it."

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An area about 50m back from the beach was used from the 1970s until the 1990s as a rubbish dump.

Over the years the rubbish has been blown closer to the beach, and covered by sand.

"The dump was closed about 20 years ago, but now it has moved closer to the beach."

A bit of metal was among the rubbish, but the biggest issue was the plastic bags, he said.

"It just amazes me: they have been in there 30-odd years, and not degraded at all."

Kokshoorn said a planned working bee had been cancelled and instead contractors with diggers would start removing the rubbish tomorrow morning.

The weather has settled in the wider area and the focus was now on the clean-up.

Kokshoorn said at least one house had lost a roof and a derelict hotel in the central business district may have to be pulled down as it was a safety hazard.

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Revingtons Hotel, which was already earmarked for demolition, was badly damaged in the strong winds yesterday and part of its roof was ripped off, letting the wind in below it.

Kokshoorn said the building posed a danger to the public, as it was in one of the town's main areas, and should be brought down as soon as possible.