A family is devastated after their 100-year-old cottage on the Banks Peninsula was knocked off its foundations and partly destroyed by a tsunami wave.

Edward and Penny Aitken's historic homestead at Little Pigeon Bay was found to be a write-off after being hit by a tsunami triggered by Monday morning's magnitude-7.5 earthquake.

The cottage is the only house on the inlet, on the northern side of the Banks Peninsula.

The Aitkens said they rented the property out, and it would've been "very scary" if someone was in it when the wave hit.

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"We had a call from the skipper of a marine barge, saying he had seen a bit of furniture floating out in Pigeon Bay so that gave us a bit of warning, and we drove out here and saw the mess that the tsunami had created.

"It's been rented to lots of families over the last 30 years so it would've been very scary if it had happened then, especially in the dark.

Several walls and a verandah have been destroyed, and furniture and appliances had been damaged.

"The waves just came in ... and surged right up ... 150m up the valley. It must've been quite powerful to push the front of the house in and cause the damage that it has," Edward Aitken said.

He hopes to demolish the house and rebuild it.

"We [hope] we can rebuild it with a slightly higher elevation and we can get back to renting it out to those Canterbury families who have been coming here all those years."