A Dunedin geologist, ignoring the vibrations of a major earthquake slapping against the hull of his boat, grabbed his camera and shot pictures of Christchurch cliffs falling into the sea.

Andy Winneke was sailing his 20ft yacht off the Christchurch coast near Godley Head yesterday when the 5.7 magnitude quake struck about 1.13pm.

He said feeling the quake aboard the yacht made him and his friend check whether they had been hit by a marine mammal or run over some debris.

"We felt the vibration under the boat, which was sort of startling.

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"It felt as if the boat had rumbled over some floating debris," Mr Winneke, a geologist with Macraes mine owner Oceana Gold, said.

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The pair "simultaneously" grabbed a camera and cell phone, one taking photos of the cliff face falling into the water and the other ringing friends to see if they were OK.

"We were just immediately concerned for Christchurch. We made a call to some friends in Christchurch to make sure this wasn't absolutely calamitous."

Another yacht, sailing close to the cliff face when the quake happened, caused a moment of anxiety.

"There were a series of cliff collapses all along the peninsula. We could hear falling rock.

"It sort of almost appeared these guys were sailing out from it at the time. They would have been checking their pulses."