But the office blocks shattered when the unusual force of last week's shallow quake hurled them upwards, as well as side-to-side, before dropping them.

The Canterbury Television (CTV) and Pyne Gould Corporation buildings tumbled in Tuesday's 6.3 quake, killing dozens including more than 60 Asian language students predominantly from Japan and China and their college staff.

New Zealand has had to promise Japan and China it would "vigorously" probe the collapse of the CTV building in which the students died, as it battled to identify the victims.

City engineer Steve McCarthy said a number of city engineers had given both buildings a green sticker, deeming them safe, after a more powerful 7.0-magnitude tremor rocked Christchurch last September.

"Our belief is that they had structural engineers in and I'm confident that the engineers would have disclosed if there was a problem with the buildings," he told reporters.

But while it was smaller in magnitude, Tuesday's quake was shallower and closer, transmitting more energy to the city centre, and McCarthy said it caused even the most modern buildings to fail.

"The unique thing about the earthquake as well was that it vertically lifted the ground and buildings and then dropped them back on the ground at two times the force of gravity.

"Consequently the buildings have failed, sadly, and it couldn't have been expected and it certainly wasn't designed for.

"Essentially Mother Nature dealt a blow that our buildings were not able to cope with," McCarthy said.

The CTV building was constructed during the 1980s and the Pyne Gould building in the 1960s.

Since the earthquake city officials have inspected 1,750 buildings in the city centre, 640 of which -- or 36 percent -- have been "red-tagged", or essentially condemned, while another 394 were declared at risk.

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