More buildings came down and there was severe disruption to electricity and water supplies, with Christchurch still recovering from the destruction of the deadly 6.3 earthquake in February which killed 181 people.

"We are in a serious situation," said Bob Parker, mayor of New Zealand's second-largest city.

The National Crisis Management Centre controller David Coetzee said there were no reported fatalities from the latest tremors but at least 10 people were injured and more buildings in the central business district collapsed.

Power company Orion said electricity had been cut to 54,000 homes and council staff said they had lost water pressure across the city.

The US Geological Survey measured the largest quake at a magnitude of 6.0, with a depth of nine kilometres (5.6 miles) and the epicentre some 14 kilometres from the heart of the city. It hit at 2:20pm (0220 GMT).

It followed a string of shocks early Monday, including a 5.2-magnitude jolt which caused building damage in the city, and came just as schools were about to end for the day. All schools have now been closed until further notice.

"It will be a restless night," Parker warned. "It's pretty realistic to expect overnight we will have some more aftershocks.

"If you don't have power and don't have water at the moment assume you won't have them overnight."

Police confirmed at least one building had fallen over in central Christchurch, which remains blocked off after the February 22 earthquake levelled much of the city, but said nobody was trapped.

"We've checked the collapsed building and it's all clear," a police spokesman told AFP. The size of the building was not known.

In an outer suburb a block of shops vacant since being damaged in February also collapsed, as did the historic 134-year-old Timeball Station -- which used to indicate the time to ships -- in the port area of Lyttelton.

The 6.0 quake prompted police to evacuate parts of the damaged central city known as the red zone, while rock falls closed several bridges and rocks littered the roads in hill areas.

The central police station was temporarily evacuated until it was declared safe by engineers.

Police urged residents to check on friends and neighbours, and to stay at home and avoid travelling if possible, but locals gridlocked the roads as they attempted to find their way home and reach their families.

The series of quakes came just hours after the opening of an inquest examining why an office block collapsed in February's earthquake, killing more than 100 people, including 65 foreign students.

When the first tremors hit around lunchtime, lawyers and relatives of those killed when the Canterbury Television (CTV) building toppled and then burst into flames on February 22 fled the building as windows rattled.

"They're a terrible reminder," said Mike Barry, whose sister was killed in February.

The inquest was briefly suspended twice by the ongoing earthquakes before being adjourned for the day after the 6.0 shake.

Geoscience Australia seismologist David Jepson said people would have been quite strongly shaken after the seismic activity New Zealand's South Island had endured in recent months.

"It seems like they are going to keep happening. Since they had that really big earthquake it seems to me like it activated all these different faults. But it's just a guessing game what is going to happen next," he said.

One Christchurch city worker, who identified himself only as Bryce, said he was pleased he was about to move to Auckland.

"It's three days till I leave this earthquake-ridden town. I can't wait."