The 7.1-magnitude quake has already been followed by a magnitude 6.2 aftershock and there will be more, GNS warns.
The first quake struck at 4.40am and was 130km north-east of Te Araroa and 55km deep. Since then there have been dozens of aftershocks.
Civil Defence earlier issued a tsunami warning, which has now been cancelled.
GNS Science seismologist Bill Fry said there was an increased chance of other big earthquakes striking near the region.
