WELLINGTON (AFP) - Emergency crews launched a major clean-up on Monday following a 6.5-magnitude earthquake in Wellington, as aftershocks continued to rattle the New Zealand capital.

The quake, the strongest of scores of tremors to hit central New Zealand since Friday, sent masonry and glass plunging into city streets as panicked locals bolted for cover.

It struck at 5.09 pm (1.09pm Singapore time) on Sunday 57 kilometres south-southwest of Wellington at a depth of 14 kilometres, the US Geological Survey said.

The city's downtown area was eerily quiet on Monday morning, with authorities urging office workers to stay at home so emergency crews could carry out damage assessment and clean up debris.

Chunks of fallen masonry were visible on some pavements and Mayor Celia Wade-Brown said there were fears more glass could drop from office towers which were sent swaying by the seismic jolt.

Ms Wade-Brown said there were only four reports of minor injuries from the quake which occurred when the central business district was largely empty.

"We're very grateful that that's how we've come out of quite a significant earthquake," she told reporters.

However, nerves remained frayed in New Zealand, where a devastating tremor two years ago killed 185 people in the main South Island city of Christchurch.

"I worry about the mental impact on Wellingtonians, just the fear factor," Prime Minister John Key told TVNZ.

"People have high levels of anxiety in earthquakes, that's totally understandable, they feel helpless, they can't control the situation."

The official GeoNet seismic monitoring service described the quake as "severe" and said there was a one-in-three chance of another tremor measuring 6.0 or above in the next week.

It said dozens of aftershocks were recorded in the region overnight, the strongest measuring 4.9.