CHRISTCHURCH, New Zealand (AP) -- The siblings huddled Wednesday on sodden grass, staring at the smoldering remains of a building that collapsed with their mother inside.

They hadn't heard from TV presenter Donna Manning since a powerful earthquake tore through one of New Zealand's largest cities, killing at least 75 people and leaving some 300 missing in the rubble. Still, there was hope.

"My mum is superwoman, she'd do anything," Manning's 18-year-old daughter Lizzy said, tears streaming down her face.

Just then, a police officer approached and knelt before Lizzy and her 15-year-old brother Kent in the rain. "I have some horrible news..." the officer began.

The teens' faces crumpled, and their father wrapped them in an embrace. There was no hope left for anyone trapped inside the building, the officer said gently.

It was one of the darkest moments of a desperate hunt for any signs of life in the twisted rubble in the city of Christchurch, as Prime Minister John Key declared the quake a national disaster and analysts estimated its cost at up to $12 billion.

Hundreds of troops, police and emergency workers raced against time and aftershocks that threatened to collapse more buildings. They picked gingerly through the ruins, poking heat-seeking cameras into gaps between tumbles of bricks and sending sniffer dogs over concrete slabs.

More teams rushed in from Australia, Asia, the United States and Britain, along with a military field hospital and teams to help repair power, water and phone lines that were damaged in all corners of the city of some 350,000 people.

The news was grim at the Canterbury Television building, a seven-story concrete-and-glass structure that housed the regional TV network where Manning was a morning presenter and other businesses, including an English language school used by young visitors from Japan and South Korea.

The heavy concrete floors lay piled atop one another Wednesday, its central stairwell tower still standing, but leaning precariously.

"We don't believe this site is now survivable," police operations commander Inspector Dave Lawry told reporters, announcing that rescuers were shifting to sites that were less dangerous and where there was more hope for survivors.

Canterbury TV chairman Nick Smith said 15 of his employees were still missing and assumed inside the collapsed building. Ten Japanese language students were still missing from a group of at least 23 students and teachers who were believed in the building, said Teppei Asano, an Japanese official monitoring the situation.

Not far away, cheers erupted Wednesday as rescuers pulled a woman from another crumpled office tower. Ann Bodkin was reunited with her husband after a painstaking rescue from the twisted metal and concrete remains of the Pyne Gould Guinness building. Coincidentally, giant sunbeams burst through the city's gray, drizzly weather as she emerged.

"They got Ann out of the building, and God turned on the lights," Christchurch Mayor Bob Parker said.

Many sections of the city of 350,000 people lay in ruins, and police announced a nighttime curfew in a cordoned-off area of downtown to keep people away from dangerous buildings and to prevent opportunistic crime.

Six people had been arrested since the quake for burglary and theft, said police Superintendent Dave Cliff, announcing that anyone on the streets after 6:30 p.m. without a valid reason could be arrested.

One of the city's tallest buildings, the 27-floor Hotel Grand Chancellor, was showing signs of buckling and was in imminent danger of collapsing, Fire Service commander Mike Hall said. Authorities emptied the building and evacuated a two-block radius.

Parker said 120 people were rescued overnight Tuesday, while more bodies were also recovered. About 300 people were still unaccounted for, but this did not mean they were all still trapped, he said.

Key, the prime minister, said early Wednesday that the death toll stood at 75 and was expected to rise. The figure had not been updated by nightfall.

The true toll in life and treasure was still unknown, but the earthquake already was shaping as one of the country's worst disasters.

JP Morgan analyst Michael Huttner conservatively estimated the insurance losses at US$12 billion. That would be the most from a natural disaster since Hurricane Ike in 2008 at $19 billion, according to the Insurance Information Institute.

Rescuers who rushed into buildings immediately after the quake found horrific scenes.

A construction manager described using sledgehammers and chain saws to cut into the Pyne Gould Guinness building from the roof, hacking downward through layers of sandwiched offices and finding bodies crushed and pulverized under concrete slabs.

One severely trapped man passed away after talking awhile with rescuers, Fred Haering said.

Another had a leg pinned under concrete, and a doctor administered medicine to deaden the pain. A firefighter asked Haering for a hacksaw. Haering handed it over and averted his eyes as the man's leg was sawed off, saving him from certain death.

"It's a necessity of the game," Haering said Wednesday. "How are you gonna get out?"

The quake struck just before 1 p.m. local time on Tuesday, when the city was bustling with commerce and tourism. It was less powerful than a 7.1 temblor that struck before dawn on Sept. 4 that damaged buildings but killed no one. Experts said Tuesday's quake was deadlier because it was closer to the city and because more people were about.

Christchurch's airport reopened Wednesday, and military planes were brought in to fly tourists to other cities.

Officials told people to avoid showering or even flushing toilets, saying the damaged sewer system was at risk of failing. School classes in the city were suspended, and residents advised to stay home.

Christchurch's main hospital was inundated with people suffering head and chest injuries, said spokeswoman Amy Milne. But officials said the health system was coping, with some patients moved to other cities.

Tanker trucks were stationed at 14 spots throughout the city where residents could come to fill buckets and bottles, civil defense officials said, and people asked to catch and save rainwater.

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Associated Press writers Steve McMorran and Ray Lilley in Wellington, New Zealand, and Kelly Doherty in Sydney contributed to this report.