No serious damage or fatalities were reported in the Valentine’s Day quake that struck at 13:13 local time (00.13 GMT).
Beaches to the east of Christchurch were busy with swimmers and surfers when rocks began to fall into the sea.
The 5.7 magnitude quake occurred days before the anniversary of a deadly one in 2011.
That destroyed the city centre and killed 185 people.
New Zealand lies on the notorious Ring of Fire, the line of frequent quakes and volcanic eruptions that circles virtually the entire Pacific rim.
New Zealand’s seismological body GNS Science classified Sunday’s earthquake as “severe”, recording that it took place at a depth of 15km (nine miles) and 15km to the east of the city.
Resident Stephen O’Dwyer said he was on the beach with his wife walking their dog when the cliff in Sumner started to collapse.
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