CTV building survivor Kendyll Mitchell held back the tears as she told the royal commission her remarkable story of how she and her two young children survived a terrifying five-storey plunge in February last year.

Ms Mitchell was on the top floor of the doomed Christchurch building waiting to undergo counselling for her three-year-old son Jett's anxiety, brought on by the September 2010 earthquake, when the fatal February 22 quake struck at 12.51pm.

She grabbed Jett and her 10-month-old daughter Dita's buggy as the concrete building pancaked in the violent shaking.

After being unconscious for 10 minutes, Ms Mitchell woke to her children staring at her.


But they were now trapped in a metre-square rubble coffin.

"I thought, 'Oh my god, we have survived, and now this','' she told the Canterbury Earthquake Royal Commission today on a dramatic opening day of the hearing into the collapse of the CTV building.

Other survivors of the collapse, which claimed 115 lives, also gave evidence today as the commission tried to establish a clear picture of events leading up to, and following the single biggest site of tragedy in last year's quake disaster.

Nilgun Kulpe was working on the top floor as a counsellor at Relationship Services - dealing with victims of quake stress, like young Jett - when the quake hit.

She described the collapse as like "being in an elevator'', and being "completely gobsmacked'' to discover that she just had plummeted five stories.

Before the hearing opened, commission chair Justice Mark Cooper acknowledged the bereaved families and expressed his "deepest sympathy'' to them.

He also apologised for what might seem a "cool and dispassionate'' process as the commission looks at how the concrete building failed so catastrophically when the magnitude 6.3 shake struck.

In his opening statement, commission lawyer Stephen Mills QC gave an overview of what to expect over the next eight weeks.

Dozens of witnesses will be called as the commission probes the causes of the building failure.

The hearing will look at the permit process, the design and construction phases, a close examination of the code compliance, remedial measures carried out after faults were found in 1990, and the assessment process which started on the building after the September 4 quake.

But one key witness, who oversaw the construction of the Madras St office block in the mid-1980s, has refused to give evidence, it emerged today.

Gerald Shirtcliff had "ultimate responsibility'' of ensuring it was built to comply with the drawings and calculations he was given. The royal commission has traced the former Williams Construction manager in Australia but has refused to give his exact location.

Mr Shirtcliff has been "advised formally'' that the Department for Building and Housing (DBH) technical report into the collapse, released earlier this year, has identified significant defects, some of which "may have played a role in the building's collapse.''

Mr Mills said to expect differing views from experts over how much damage the building suffered after the September 4, 2010 quake, as well as from structural engineers over responsibilities for the building design.

A dispute between structural engineer David Harding and his boss at Alan Reay Consulting Engineer, now operating as Alan Reay Consultants, was likely to be heard.

Mr Harding, who was "inexperienced'' in designing multi-storey buildings, would refute claims that Dr Alan Reay, principal engineer at his own Christchurch firm, spent just three and-a-half hours working on the design.

The royal commission would not attribute blame for the building's failure and questions of legal liability would not be addressed, Mr Mills said.

It will report on the causes of building failure as a result of the earthquakes as well as the legal and best-practice requirements for buildings across New Zealand CBDs.

The royal commission has until November 12 to deliver its final report.