National Civil Defence headquarters says the major earthquake off the south Taranaki coast on Tuesday was on the threshold at which authorities consider whether it should be upgraded to a national event.
GeoNet says the magnitude 7.0 earthquake struck at 10.36pm on Tuesday. It was centred out to sea, 60 km south of Opunake and 170 km northwest of Wellington.
The quake was felt from Napier to Dunedin, but there have been no reports of serious damage.
Residents in the North Island have described it as a long rumbling sensation and a jolt, followed by more rolling.
A magnitude 4.6 aftershock occurred in the same area at 10.45pm, centred 70 km south of Opunake and 160 km north-west of Wellington.
The quake reached level 6 on a scale measuring the intensity by which it was felt. That triggered the National Civil Defence headquarters to look at whether a tsunami was likely, and whether the quake had caused significant damage.
Taranaki Regional Council senior emergency manager Shane Briggs says it very quickly became clear that a tsunami was unlikely and damage was likely to be minimal.
Even so, he says, some people did move away from the coastline, and it was still a serious shake for Taranaki, where earthquakes are rare.
GeoNet said the earthquake was the biggest in the region since a 6.4 earthquake on 15 March 2005 and was a result of the Pacific Plate being driven under the Australian Plate.
Seismologist Lara Bland said that because of the depth of the earthquake its energy travelled efficiently up through the subducting plate.
"The result has been that it's been felt very, very widely across central New Zealand areas."
She said the impact of the earthquake would have been quite different had it been shallower.
"When you look at the Darfield quake in September 2010, that was a very, very similar magnitude, and we saw what that did, being so much shallower."
Darfield was close to the epicentre of a 7.1 earthquake on 4 September 2010 that was 10km deep and caused destruction over a wide area. It was followed by hundreds of aftershocks.
Ms Bland says there is no connection or triggering effect between Tuesday's earthquake and the Canterbury earthquakes.
A magnitude 7.0 earthquake occurs in the New Zealand region on average once every three years, GeoNet says.
Reported damage minor
Shell Exploration says the 7.0 magnitude quake did not damage the Maui A and Maui B drilling platforms off the Taranaki coast.
In Wellington, water tanks overflowed in the 14-storey TSB building on the corner of Lambton Quay and Waring Taylor Street, causing damage to the top two floors.
Some mobile phone users said they had no reception in Wellington following the earthquake but both Telecom and 2degrees said there was no reported disruption to service.
Circuit breakers at Radio New Zealand's transmitter in Titahi Bay knocked out broadcasts on the AM network in the Wellington area for 45 minutes during the shake.