The quake, which struck at sea 60 kilometers (38 miles) west of Milford Sound on South Island's west coast at 1:29 a.m. local time Tuesday (1229 GMT Monday), was some 24 kilometers (15 miles) beneath the surface, New Zealand geological agency, GNS Science reported on its Web site.
The Pacific Tsunami Warning Center in Hawaii issued an information bulletin stating that it did not expect a destructive tsunami to be generated by the quake.
Police senior sergeant Bruce Ross in the southern city of Dunedin said, "We felt it (here), but there are no reports of injury or damage."
"We heard the blinds rattle a bit, but that was it," he added.
Inspector Alan Weston in the southern city of Christchurch said about 10 people had called shortly after the quake, but "we have no reports of damage or any injury," adding the sharp quake was "not felt in Christchurch."
New Zealand sits above an area of the earth's crust where two tectonic plates are colliding and records more than 14,000 earthquakes a year - but only about 150 are felt by residents. Fewer than 10 a year do any damage.
The choice of the city of Helsinki is not incidental as the capital of Finland had hosted US-Soviet negotiations on the limitation of nuclear stockpiles in 1969