USGS said the quake was centered 180 kilometres east of Raoul Island at a depth of about 24 miles. The islands lie about 1,200 kilometers north of the North Island.
There are no reports of damages so far and the Hawaii-based Pacific Tsunami Warning Center said there was no tsunami warning in effect for coastal areas.
But it warned that some areas in the region may experience small non-destructive sea level changes lasting up to several hours.
New Zealand, currently hosting the Rugby World Cup, lies on the Pacific "ring of fire", a zone of frequent seismic activity, and is prone to earthquakes.
The second largest city, Christchurch, was devastated by a 6.3-magnitude quake in February which killed 181 people, after many of its buildings were weakened by a previous earthquake in September 2010.
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