Image caption The building collapsed within 10-20 seconds of the onset of the quake

A building that collapsed in New Zealand's Christchurch earthquake, killing 115, was badly designed and should not have been approved.

The report from the Royal Commission said the engineer who designed the Canterbury Television (CTV) building was working "beyond his competence".

His overseer took a hands-off approach and did not review structural details of the project, it found.

Prime Minister John Key described the report as "grim" reading.

'Little comfort'

The Canterbury Earthquakes Royal Commission released the third and final part of its report after more than two months of hearings focusing on the CTV building.

It concluded that the six-storey structure collapsed within 10-20 seconds of the onset of the magnitude 6.3 earthquake on 22 February 2011.

Almost two-thirds of the 185 people who died in the natural disaster were inside the CTV building.

"The engineering design of the building was deficient in a number of respects," it found.

"While there were elements of the applicable codes that were confusing, a building permit should not have been issued for the building as designed."

The report said that the overseer should have realised that the design pushed the engineer "beyond his limits given his past experience".

"The design process led to a building that was under-engineered in a number of important respects," it said.

A number of construction defects had also been identified, the report said, adding that the construction manager "did not spend sufficient time on the site to perform his role adequately".

In its report the commission gave six specific reasons for the collapse, five of which related to the design or construction of the building. The sixth was the intensity of the quake itself.

It also said that post-quake assessment procedures should be improved, highlighting the fact that the CTV building was approved for use after the September 2010 earthquake without being examined on a structural level.

In a statement, Prime Minister John Key said that the commission's report made for "grim and sobering reading".

"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.

Describing the commission's full seven-volume report as "incredibly complex", he said the government would take the time needed to consider the recommendations and "expects to issue its full and comprehensive response by early to mid-2013".