The Canterbury Television (CTV) and Pyne Gould Corporation buildings tumbled in last Tuesday's 6.3-magnitude quake, killing dozens, including more than 60 Asian language students predominantly from Japan and China, and their teachers.
City engineers deemed both buildings safe after a major tremor last September which miraculously resulted in no deaths in Christchurch, a result attributed at the time to New Zealand's stringent building standards.
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Key said there were legitimate questions about why the buildings were reduced to rubble in last week's earthquake, which killed at least 155 people and is expected to result in a final death toll of more than 240.
Relatives deserved an explanation about why their loved ones perished, he said, promising a thorough inquiry into both the fate of the office blocks and New Zealand's building codes.
"Obviously the earthquake was something that's beyond our control (but) we are going to get answers for those families about what's gone wrong," he said.
Key said the CTV building, which housed the English language school, was "a place where far too many people have lost their lives". The deaths of so many young students from foreign shores was keenly felt in his country, he said.
"We know that they were entrusted to the people of New Zealand," he added.
New Zealand has promised Japan and China that it would "vigorously" probe the collapse of the CTV building, which was engulfed by flames after the quake, meaning remains of many of the dead at the site may never be identified.
The building was constructed during the 1980s, while the Pyne Gould office block went up in the 1960s.
