New Zealand Prime Minister John Key renewed his pledge Monday that the country 's second largest city of Christchurch would be rebuilt after a series of fresh tremors rocked the city, injuring dozens of people.

The government-run GeoNet recorded 21 tremors in Christchurch from 1 p.m. to 8.45 p.m., with the most violent quake being a magnitude 6 at a depth of 9 km, centered about 10 km southeast of the city, at 2.20 p.m..

The second worst was a magnitude 5.5 quake at 1 p.m. with a depth of 11 km, centered about 10 km east of the city.

The quakes, which left at least 46 people injured, came after the Feb. 22 quake which killed 181 people and another in September last year that caused severe damage in the South Island city.

"It's another blow to the Christchurch residents who have already endured so much," Key told a press conference.

"However today's events in no way weaken the government's long term commitment to rebuild Christchurch and surrounding areas."

SAPPING CONFIDENCE

Key said he had spoken to Mayor Bob Parker, who was considering whether to call a state of emergency. Earthquake Recovery Minister Gerry Brownlee would be flying from Wellington to Christchurch as soon as possible, the National Business Review reported.

Key said the situation was "not considered serious enough" for a national state of emergency to be imposed.

The quakes were a "setback" that caused further damage to the central business district and the eastern suburbs, cutting electricity to thousands of homes and water supplies and damaging sewer systems.

"For the people of Christchurch I am sure they just want this to end," he said.

"I am sure they are over all of this and they want a sense of normality to return and I think we can all feel their frustration. But they've got to know that we stand beside them, that we are totally committed to rebuilding the city and I am sure these aftershocks will eventually settle down and normality will return. It's a very frustrating time for the people of Christchurch."

"That saps away the confidence of the people of Christchurch."

MORE TO COME

The government's GNS Science warned that further strong aftershocks could follow in the next few weeks.

"This size of events is likely to produce its own aftershock sequence, therefore rejuvenating aftershock activity at least in the short term. We would expect a number of aftershocks in the magnitude 4 to 5 range on the coming days and weeks," said Kelvin Berryman, manager of the GNS Natural Hazards Research Platform.

The tremors on Monday disrupted an inquiry into the deaths of 116 people killed in the CTV building during the Feb. 22 earthquake.

The coroner's inquest had just broken for lunch when a 5.5- magnitude shock truck at 1 p.m., and family members and media were evacuated from the venue.

The inquest had been hearing how 12 people inside The Clinic medical practice in the CTV building died.

COLD AND FLOOD

The Ministry of Education has strongly advised all schools and early childhood education services in Christchurch to remain closed Tuesday so that management can assess the situation.

Households without electricity were facing "an extra cold night" Monday as temperatures dropped to freezing or below, the WeatherWatch website warned.

Engineers were racing late Monday to prevent flooding, as high tides were expected at 2.25 a.m. and 3 p.m, Tuesday and there was a risk that low lying areas could be inundated, said a statement from the city council.

Temporary stopbanks along the Avon and Heathcote rivers had been damaged. Crews are working to restore them, but it was unlikely that the work would be completed for the high tide in the morning. The council was expecting some localized flooding.

"At this stage we are not expecting tides any worse than this afternoon, however there is a need for precautions in the lower reaches of both the Avon and Heathcote," said the statement.

Flooding occurred in parts of the city Monday, and heavy flooding in a couple of areas left large unseen potholes in the roads, warned the statement. Four bridges in the city were also closed.

The council warned people to boil water before drinking and to avoid flushing toilets.

The quake also disrupted flights into and out of Christchurch airport as well as rail services around the South Island.

DEMOLITIONS NECESSARY

The Christchurch Earthquake Recovery Authority (CERA) demolition manager told Radio New Zealand that Monday's quakes had taken parts of Christchurch's inner city red zone back to what they were like in February.

Warwick Isaacs was with engineers checking buildings in Lichfield Street when the quakes struck.

The 26-story Hotel Grand Chancellor, already leaning precariously, was one of many buildings to be further damaged.

Isaacs said up to 50 additional buildings within the zone had collapsed or partially collapsed.

He said masonry and bricks fell all around him in Lichfield Street from buildings up to five stories high.

Engineers would spend four or five days carrying out assessments and emergency demolitions might have to take place, he said.

The head of the CERA told Radio New Zealand that several buildings in the central city which were in need of demolition had collapsed on their own.

"There're buildings now that are seriously weakened that weren' t actually damaged before this, said CERA chief executive Roger Sutton.

"We need to bring them down so we can safely access other areas.

"If anything it may actually speed up some aspects of the recovery rather than slow them down."

The historic Timeball Station tower in nearby Lyttleton had collapsed following the magnitude 6 earthquake, said the New Zealand Historic Places Trust (NZHPT).

While some of the building remained standing, the site was too dangerous to enter for an assessment of its state to take place.

Work had begun on dismantling the NZHPT property two weeks ago. The immediate area had already been fenced off following the Feb. 22 earthquake that severely damaged the 134-year-old iconic landmark.  

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