A group of people who tried to escape down the stairs of the 18-storey Forsyth Barr building following February's earthquake in Christchurch could have plunged into a dark void, the inquiry heard.

The Canterbury Earthquakes Royal Commission on Thursday heard evidence of the staircase collapse, as part of its investigation into the failure of buildings.

The collapse left the building's tenants stranded on their floors for several hours, and a number escaped by abseiling down the building before others were rescued by a crane, counsel assisting the royal commission, Stephen Mills QC, said.

"By great good fortune no one was on the stairs at the time, or fell in trying to descend non-existent stairs in the dark," he said.

Mr Mills mentioned the reported account of an unidentified woman, referred to as Jane, who had been on the building's 17th floor when the quake hit, and who had attempted to leave via the stairs.

She made it to the 16th floor, where five others joined her, and when they reached the 15th floor, they were advised there were no stairs left below that floor.

"Had Jane and her party continued further it is likely they would have plunged into what was by then a dark void," Mr Mills said.

The stairs in the building were pre-cast scissor stairs which were fixed at the top end of each flight, and designed to slide at the bottom end during an earthquake, the royal commission has heard.

The inquiry is due to continue on Friday opening with evidence from the site manager for the construction of the Forsyth Barr building, Paul Tonkin.