A gallery patron sitting underneath the overbridge was showered with blunt shards of glass but was apparently uninjured.
Gallery director Tony Preston said the pane of toughened safety glass inexplicably shattered about 11.35am yesterday. The area under the walkway was immediately cordoned off and members of the public escorted out of the building by staff.
The gallery, which opened in May, remained closed for the rest of the day as the architect and the glass supplier were called in to investigate.
Mr Preston said it was regrettable to have to close the facility but the safety of the public was paramount.
"We are thoroughly investigating the causes of the breakage and will take immediate steps to ensure any identified problems are rectified."
Christchurch City Council property and major projects committee chairman Ron Wright said that when the first glass panel broke the council had been assured it was a one in a million occurrence and there was nothing technically wrong with the glass or its installation.
"It's extremely concerning to me that another panel of glass has shattered," Mr Wright said.
Last night, the remaining curved glass on the overbridge was removed as a precaution.
Mr Preston said the decision to close the gallery and remove the glass was based on the fact a walkway panel had broken previously. The earlier event occurred outside opening hours.
"The gallery had investigated the cause with the suppliers and the project control group at the time, and had been reassured that it was an isolated event. However, our response to this second incident is to take the immediate public precaution of closing the overbridge and removing all similar glass panels," Mr Preston said.
The glass panels on the overbridge used a unique glazing system and did not feature elsewhere in the gallery.