The warning came from an Australian-based commercial construction quality manager in Dunedin yesterday, one day after two men - one in a wheelchair - were showered with falling glass fragments when a glass panel shattered inside the Wall Street mall.
The manager - who would not be named - told the Otago Daily Times the failure seemed to be a case of "nickel sulphide inclusion", which could occur during the manufacture of float glass that is later toughened.
Nickel contamination meant nickel sulphide crystals, formed during glass toughening, slowly reverted to a larger form, expanding to trigger shattering.
In Australia, industry rules now recognised the risk by requiring toughened glass to be heat-soaked and laminated before use, he said.
Heat-soaking identified faulty toughened sheets by making them fracture. Laminating prevented any toughened glass that shattered from falling to the ground, he said.
Without either measure, there was a risk more glass panels could shatter and collapse.
Visual inspections would not detect the fault, he warned.
"They need to actually change that glass over, because there is a potential risk for those skylight panels to spontaneously combust [shatter], as that particular panel has.
"What has happened there could quite easily happen again," he said.
It could also place lives at risk, as even safety glass that shattered had the potential to kill, he believed.
That was evident from the dent left in the roof of a Mazda display vehicle, parked inside the mall on Wednesday, which was also hit by the falling glass fragments, he said.
"It [safety glass] is designed to break apart, but it can actually be held together in large pieces that have shattered.
"One of those pieces could easily kill someone."
The Australian-based manager - a former Dunedin man in the city on holiday - said Wednesday's incident had highlighted a design error, as laminated and heat-soaked glass should have been used.
"Clearly what's happened there is totally inadequate."
Dunedin City Council city property manager Robert Clark, alerted to the man's concerns yesterday afternoon, said they would be taken "very seriously".
Mr Clark said the investigation was continuing and he would meet the Australian-based manager this morning to discuss his concerns.
It remained possible the glass failure was a one-off, "but we don't know that until we have done a double-check on that particular panel," he said.
"We are concerned about public safety and we will do whatever has to be done."
Mr Clark later emailed a report from Parker Warburton Team Architects Ltd - the lead architect in Wall Street's construction - to the ODT.
The report by Parker Warburton director Simon Parker, of Dunedin, said nickel sulphide inclusion was a "worldwide problem" that could affect 1 in 2500 sheets of glass, but 90% of those exploded before leaving the factory.
The council-owned $34 million shopping mall opened to the public in March 2009.
Mr Parker said laminated glass had been considered during earlier design work, but ruled out because of its weaker strength and the "much worse" potential for thermal stress cracking.
Heat soaking was a "relatively new process" in New Zealand and was not considered in 2007, but the glass used met New Zealand standards and had performed as designed, he said.
The mall was closed immediately after Wednesday's incident, but reopened - with the public atrium cordoned off - after council maintenance staff decided the remaining glass panels were safe.
Mr Parker said glass contractors were to remove remaining pieces of the shattered panel last night, install a replacement next week, and continue an investigation into the exact cause.
Nickel sulphide inclusions in failed toughened glass are typically only confirmed by using a scanning electron microscope.
Dunedin support worker Ed Bedford was accompanying a wheelchair-bound client to the mall when the 2m-long glass panel shattered at 11am, showering both men with glass fragments.
Mr Bedford told the ODT the shattering of the glass was "like a bomb going off".
