WELLINGTON (REUTERS) - An earthquake with a preliminary magnitude of 6.8 struck south of New Zealand's capital, Wellington, on Friday, sending panicked workers and residents into the streets just weeks after a similar size quake struck the city.

The quake, which hit near the northern tip of New Zealand's South Island, was at a depth of about 10 km, according to NZ Geonet, which originally gave it a magnitude of 6.0.

While fire authorities said it was too early to assess the impact, there were some reports of superficial damage to buildings from the quake, which did not cause a wider tsunami alert, but sent items tumbling from shop shelves.

Air and rail services were being suspended while officials checked tracks and runways for damage.

There were also widespread power outages across the north of the South Island. There was no specific threat of a widespread tsunami, according to the Pacific Tsunami Warning Center.

New Zealand has been hit by a string of quakes since a shallow, 6.3 magnitude tremor devastated the South Island's Canterbury region in 2011, killing nearly 200 people and levelling Christchurch, the country's second largest city.

Earthquakes are common in New Zealand, whose two islands lie along the Australia-Pacific tectonic plate boundary.