Cutting through the rubble, concrete and metal in the grim task of finding the dead is all in a day's work for almost 579 Urban Search and Rescue (Usar) personnel on the ground in Christchurch.

Seven international teams with 429 Usar experts have descended on central Christchurch since Tuesday's earthquake, working around the clock to look for any signs of life and, in the past days, locating the dead.

They have joined the 150 New Zealand Usar personnel.

"Conditions are very severe," said the head of the Queensland Usar team, John Cawcutt.

"The boys are getting very little sleep, eating ration packs. It's hot, we're doing hard, physical work, so they're very hard working conditions."

The team from Britain has pulled four bodies from the Pyne Gould building in the past two days.

"There are certainly victims there and we've located several others and we have intelligence to say there are others," said British team leader Peter Crook.

But Mr Crook said his team had been part of operations that had pulled survivors out of rubble several days after an earthquake.

"In Haiti, the rescues were three, four days in to the mission.

"We have to operate on that assumption [that people are still alive]. It's a remote chance, but there's still a chance."

He praised the New Zealand effort.

"The organisation has been outstanding, the best-organised emergency I've been to. At the start of the Pyne Gould operation, they went through four floors of concrete to get to places. It was amazing."

The British team is now using heavy machinery to make headway with the reinforced concrete, and then lifting personnel to the coal face in cranes to keep them safe.

"If there's anything there, we can find it," Mr Crook said.

"We have cameras and we've been using those to get in and see. It's obviously full of debris, glass, insulation, everything."

The team is systematically going through the building from top to bottom, front to back, as others are doing at the CTV and Christchurch Cathedral sites, though progress at the latter has been hampered by falling masonry.

Of 964 buildings in the CBD that have been assessed, 569 are safe to access, 172 are hazardous, and 223 cannot be entered because of the risk of collapse.

The head of the Singapore team, Lieutenant-Colonel Ling Kok Yong, said some of the 51 buildings they have checked were still unstable.

"It's challenging, but our rescuers are experienced.

"The Disaster Assistance Response Team members have participated in 12 overseas rescue mission since 1990."

Mitchell Brown, head of the New Zealand effort, said the teams would continue to search until no stone was left unturned.

"We always hold out hope," he said. "And until such a time as we've cleared every building and no persons are left unaccounted for, it's a rescue operation."

NZ search and rescue personnel

international personnel

from China

from Taiwan

from Britain

from United States

from Queensland

from NSW

from Singapore

from Japan (approximate number)

Plus:115 tonnes
of equipment

CBD buildings assessed

to be demolished

are hazardous

good order