The initial earthquake was centered 6 miles east of Seddon, and it hit at 2:31 p.m., the U.S. Geological Survey said.
Many aftershocks followed, TVNZ reported, with one 6-magnitude shock coming at 5:31 p.m.
A house also collapsed in Nelson, about 52 miles from the epicenter, TVNZ said.
Some 250 customers in Wellington, a city 50 miles from the epicenter where a 5.6-magnitude aftershock hit, are without electricity.
Witnesses in Seddon said the earthquake caused rock falls near the state highway, which is closed near the town.
Train services have been suspended in Wellington, and the airport also has been closed temporarily.
Two people have been treated at a hospital for minor injuries, but so far, there are no further reports of casualties.
Although there have been a series of moderate earthquakes in New Zealand in recent weeks, the epicenter of those quakes were in the water, the U.S. Geological Survey said.
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