Auckland City Council inspectors were yesterday assessing the safety of a central city building after a pane of glass fell 12 storeys to busy Wyndham St while many office workers took their lunch break.
The broken window was about 1m by 1.5m and is the third to fall from AMI House in a year.
Council building inspectors were at the site late yesterday.
Council spokeswoman Charlotte Haycock said the council was "being proactive because it was a safety and building issue".
The building manager, Bob Rogers, said the reason for the accident was not known but he would be looking into it.
"Since it happened last time, we've had the windows looked at."
He said no issues were apparent after the inspection.
"We have the windows cleaned on a regular basis and the cleaners advise us if there are any problems, but it's a bit of a mystery."
Mr Rogers said the previous breaks happened in the summer when the weather was hot and it was thought that could have been a cause.
But he said that was unlikely to be the problem this time.
A policewoman at the scene, who declined to give her name, said no one was injured.
"We're very lucky."
Senior Station Officer Barry Fox said firefighters had no idea how the window was broken.
"There was no human involvement, maybe it was some sort of move-ment."
Some passers-by ignored the safety cordon and walked underneath the building with their bags over their heads.