A number of foreign students could be among the dead in an office building that collapsed in New Zealand's earthquake, police said today.

Officers added that they are "100% certain" no-one trapped in the rubble is still alive.

Authorities called off rescue operations at the Canterbury Television building, where an unknown number of people were working when the quake struck on Tuesday.

Those trapped were thought to include as many as 10 Japanese students attending a language school on the premises and 15 employees from a TV station.

"We don't believe this site is now survivable," police operations commander Inspector Dave Lawry told reporters, adding the tower of the building that remains standing is in danger of collapse, weakened by quake aftershocks and fire.

An unspecified number of overseas students in the building when it toppled have not been located, he said.

"At a certain point, I'm not going to risk my staff (searching) for people who I believe have no chance of survivability," he said. "That's the end of it."

Canterbury TV chairman Nick Smith said 15 of his employees who worked in the building were missing.

"We're working on the assumption that everyone we haven't managed to contact was in the building, and that would number probably 15," he said.

He said staff who managed to get out of the building described a scene that was "like out of a horror movie" and broke down when recounting their ordeal.

"(They've) lost a lot of friends, a lot of colleagues, a lot of talent and a lot of lifelong relationships," Mr Smith said.

At the time of the quake, a group of Japanese students from the Toyama College of Foreign Languages was eating lunch at a language school called King's Education, said Hisao Yoshida, the college's head. According to King's Education Ltd's website, it is located in the CTV building.

The students were part of a larger group of Japanese students and teachers, one of whom emailed her family back home using her mobile phone to say she was trapped in a building, Yoshida said.

Two teachers and eleven students were rescued from the twisted building, while 10 others were missing, according to Teppei Asano, an official at Japan's Toyama city emergency room, which was monitoring the situation.

Some of the rescued had broken bones and serious injuries, he said.

Most of the students were 19 years old, but two were in their 60s, including one who was rescued and badly injured, according to a release from Toyama city, in western Japan.

Police earlier said more than 20 people had been rescued from the building and others had managed to escape. That search was now over.

"The sad fact is we're removing resources form this site to other sites where there is a higher chance of survivability," Inspector Lawry said.

"My heart goes out to those families ... knowing that some of their children have probably been killed in this incident. We will do the very best for your people that we can," he said.

The earthquake hit Christchurch, one of New Zealand's biggest cities, toppling tall buildings and churches, crushing buses and killing at least 75 people in one of the country's worst natural disasters.