Prime Minister John Key told a news conference after daybreak that two people were known to have been killed, the Associated Press reported.

A series of large aftershocks, some measuring more than 6 in magnitude, hit more than an hour after the biggest quake.

“If you are in a low lying area on the East Coast of the North, South or Chatham Islands move immediately to higher ground,” New Zealand’s ministry of civil defense and emergency management tweeted as the first waves were reported coming in. The Chathams are a small group of islands about 400 miles east of Christchurch, the largest city in the south island.

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"Please heed the warnings, don't go back to your homes until we've cancelled the advisory or given further information," Sarah Stuart-Black, national controller of civil defense, told Radio New Zealand.

Christchurch suffered a 6.3-magnitude earthquake in 2011 that destroyed large parts of the central city and killed 185 people.

Monday’s earthquake, which happened just after midnight local time, was centered in Culverden, about 60 miles north of Christchurch, and was 10 miles deep.

Local media reported that six-foot-high waves were rolling into Kaikoura, a coastal city north of Christchurch, and at least one building had collapsed. Phone service and the power supply in Kaikoura appeared to be cut off so the extent of the damage was not immediately clear.

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Tsunami sirens were reported blaring in the Wellington and Christchurch regions, and some highways were closed due to damage to the roads.

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On social media, people shared photos of furniture that had fallen over and cracks in roads, but most of the damage initially appeared to be relatively minor compared to the 2011 earthquake.

“I hope everyone is safe after the earthquake tonight,” Prime Minister John Key tweeted. “@NZcivildefence is looking into the impact of the quake. Follow them to stay updated.”

Secretary of State John F. Kerry had been in New Zealand on Sunday after visiting Antarctica, but it was not immediately clear whether he was still there when the earthquake hit.