A trained mountain guide who helped his trapped colleagues abseil six storeys to safety insists he is no hero.

John Haynes was among 15 people stuck on the sixth storey of the Forsyth Barr building at the corner of Armagh and Colombo streets.

"We put it to them: There was a way out. If they wanted to come they could. If they wanted to stay they could stay, " Haynes said.

The way out involved being lowered by rope about 20 metres onto a car park, where those trapped could escape onto the streets.

"The building was going sideways, rocking and then the stairs collapsed from the top to the bottom leaving no obvious means of getting out, " Haynes said.

"The guts of the building fell down 17 storeys. We just had a shell of a building."

Smoke and dust rushed through the building leaving 15 people trapped on the sixth floor when the quake hit.

"We met and the question was what do we do? In my mind there was a danger the building could collapse. The other was of fire."

Haynes said they could have remained "stuck there but nobody would know if we were OK or not".

"It became obvious we needed to get out, " he said.

Haynes, who works as an investigator for the Ombudsman, knew from his mountaineering experience how to get people down.

He said after the 9/11 disaster staff had installed emergency supplies, including rope, sledge hammers, axes and food in their Christchurch offices.

The trapped workers smashed a window and began preparing to descend the side of the building to safety.

Uncertain about the strength of the ropes, a former Army man "of medium size" was sent down first. "He was pretty gutsy."

The rope held and together with lawyer Grant Cameron, Haynes belayed 14 people down 3 1/2 floors to the top of the car park where people could walk down to the ground floor and escape.

It took three-and-a-half hours with only Haynes and two others then left behind.

"I said I'd get myself down last because I knew how to do that, " Haynes said.

He didn't have to. A crane driver saw the pair waving from the window and stopped "mercifully" to rescue them from the building. "That was fantastic."

Cashmere man Haynes said his colleagues were "a fantastic bunch, no histrionics" who realised it was "a life or death situation".

"I don't think I'm a hero. It was fortunate that I was able to do what I do."

But Assistant Ombudsman Christopher Littlewood, who was among those trapped, insisted Haynes was "a hero".

"John is a cool and calm fellow with huge experience in mountaineering. He's a person who rescued people from mountain sides. I knew I could trust him."