A series of nine earthquakes, two of them measuring over 6 on the Richter scale, rattled residents in the lower part of New Zealand's South Island over a period of 12 hours on Tuesday.
Seismologists with GNS Science said aftershocks from the quakes, which began with a 6.7 magnitude shock that woke up people in the island's biggest city Christchurch, 800 km away from its epicentre, at 1.30 am local time, might continue for several days.
The next biggest quake, measuring 6.2 on the Richter scale struck at 10.28 am. The others ranged between magnitudes of 4.1 and 5.8.
All were centred out to sea off Milford Sound in the island's Fiordland region and there were no reports of damage or injury.
About 14,000 earthquakes a year are recorded in and around New Zealand, one of the world's most volatile seismic zones, but only up to 150 are big enough to be felt.
The country's civil defence ministry said it had contacted the Pacific Tsunami Warning Centre, but there was no tsunami risk to Pacific coastlines from the swarm of quakes.