There are casualties following this morning's quakes, acting Civil Defence minister Gerry Brownlee says.

The extent of the casualties was unclear, and he did not yet know how many people could be injured.

He had been messaging the Prime Minister throughout the night and the New Zealand Defence Force was on standby, Brownlee said.

Speaking from the Beehive's civil defence bunker, Brownlee said the quake had been felt "extensively".

"There will be a lot of people out there very unnerved by this. The sort of holding hand you can put out to anyone who is distressed would be very welcome."

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No national state of emergency had been declared.

"Each district is making their own decision. We've been receiving into the bunker their reports. As the hours pass the situation on the ground will become more clear."

Brownlee was joined by Sarah Stuart-Black from Civil Defence and Bill Fry from GNS.

Stuart-Black said the first waves had hit coastlines, and it was likely they would not be the biggest.

People were urged to stay safe and away from the coastline.

"People should expect to feel aftershocks."