Civil Defence has defended its response time to this morning's magnitude 7.1 earthquake, saying it did trigger an alert warning.
East Cape residents have reported cracks in houses and fallen chimneys as they cope with continuing aftershocks throughout the day.
The earthquake struck at 4.40am and was centred 130km north-east of Te Araroa.
The Ministry of Civil Defence and Emergency Management sent out its first national advisory about the quake at 5.10am - about 40 minutes after the quake hit.
A potential tsunami threat notice was not issued until 5.33am and a request for an emergency announcement to broadcast was not made until almost 6am.
WeatherWatch head analyst Philip Duncan said GeoNet should issue tsunami alerts, in the same way MetService was able to issue cyclone warnings.
But Ministry of Civil Defence and Emergency Management director Sarah Stuart-Black said the initial earthquake did not meet Civil Defence's threshold for issuing an automatic tsunami warning.
"We know that the Pacific Tsunami Warning Centre had indicated that there was no tsunami threat in their initial assessment and, 23 minutes later, at 5.53am, we issued a national advisory for a potential tsunami threat, to make sure we were being proactive about that public safety consideration."
Ms Black said the quake was measured initially as a magnitude 6.3, before being upgraded to 7.1.
Another large quake possible
GNS duty seismologist Caroline Holden warned another large quake could follow this morning's jolt.
There was a rupture line of about 40km from the epicentre in the Pacific towards the coast, and therefore aftershocks could happen closer to land, she said.
There had been at least 15 notable aftershocks and people in the region should be prepared for more, she said.
However, Dr Holden said today's big quake was very unlikely to trigger quakes in other parts of New Zealand.
GeoNet, meanwhile, was investigating whether this morning's quake could have been two simultaneous earthquakes rather than just one.
It was looking at whether one quake almost instantly triggered another one on a nearby faultline.
Seismologist John Ristau said that was because the type of rolling quake that people experienced was not usually the type to generate a tsunami.
"So that makes us think that there actually might be another fault involved which involves more uplift of the seafloor."
Mr Ristau said aftershocks were likely for several months at least.
The tsunami warning from the 7.1 quake, which applied to the East Coast of the North Island, including Auckland, Waikato and the Bay of Plenty, has now been cancelled.
The quake followed a 5.7 shake yesterday about 10am.