An office block that collapsed, killing 115 people in last year's Christchurch earthquake was so badly designed it should never have received a building permit, an official report found Monday.

The six-storey Canterbury Television (CTV) building crumpled then burst into flames when a 6.3-magnitude quake rocked New Zealand's second-largest city on February 22, 2011, killing those trapped inside including 65 foreign students.

A Royal Commission found the building was poorly designed by an engineer with no experience of multi-storey structures, its steelwork was not constructed properly and that the local council failed to pick up on the problems.

As a result, the building "pancaked" within 20 seconds of the tremor hitting, accounting for almost two-thirds of the 185 deaths in New Zealand's worst quake for 80 years, the report found.

The commission outlined numerous flaws with the office block's design and concluded "(a) building permit should not have been issued" by Christchurch council allowing construction to begin in 1986.

It also said the CTV building was damaged by two quakes in the months leading up to the disaster, which levelled much of the city's downtown area, but the council allowed it to remain occupied without ordering an engineering inspection.

Prime Minister John Key said the report "makes for grim and sobering reading" and his government was considering the commission's 189 recommendations to improve earthquake safety in New Zealand buildings.

"We recognise this news will be of little comfort to the friends and families of the 115 people who lost their lives in the CTV building on that fateful day," he said.

"Nothing will ever bring their loved ones back and we cannot dull their pain. My thoughts are with them as they continue to try to come to terms with their loss."

The building housed the King's Education language school. Eight staff and 65 students, predominantly from China and Japan, died in the disaster

Its disintegration and the subsequent inferno were so destructive that forensic specialists had to use DNA testing to identify the remains of many victims.

A separate inquiry into the collapse by government inspectors concluded in February this year that the building was sub-standard and their report was passed on to police to see if further action was warranted.