The Auckland Council is investigating how a building that has been standing for 124 years can crack open in an afternoon.
The Palace Hotel building opposite the Sky Tower in Victoria St was undergoing renovation work to be transformed into an upmarket brothel when cracks appeared on Thursday.
It was demolished overnight on safety grounds.
The Herald understands the developers had consent to turn the basement - which was used only as a storage space - into a work space, which required further excavation.
A peer-reviewed engineering report is understood to have been prepared for the work.
The council said yesterday it would not make any further comment until its investigations are complete.
A spokesman for Auckland Mayor Len Brown said the mayor was expecting a report from the investigators on Monday.
The council said the cracks resulted from a failure at basement level and was investigating what caused the building to fall under its own weight.
Cracks up to 1m long could be seen in the building before the decision was made to demolish it.
"The building continued to move towards the road throughout the evening, and by midnight had moved 90mm," said council chief executive Doug McKay.
"Windows were spontaneously breaking and the building was in danger of imminent collapse."
A Birkenhead Transport employee, who did not want to be named, said he was leaning against the building as it cracked.
"I've never experienced anything like it ... the walls were cracking like an eggshell, and there wasn't even an earthquake," he said.
Historic Places Trust heritage adviser Martin Jones said questions need to be asked because "old buildings don't just fall apart like that".
The original building was built in 1886, but it was the two annexes added in 1889 and 1912 that showed signs of serious cracking.
"This is a significant loss for Auckland because there are few buildings left that date back to the city's colonial era," Mr Jones said.
The building, better known as the Aurora Tavern, was registered as a category two historic place in 1981.
The property was owned by Wellington brothel owners John and Michael Chow, who bought it for $3.3 million and planned to turn it into one of Auckland's largest brothels in time for the Rugby World Cup.
Most business owners feared that the planned brothel would turn the area into a red-light district.
The Engineering, Printing and Manufacturing Union had its origins at the tavern 147 years ago.
The then Auckland branch of the Amalgamated Society of Engineers was formed on May 29, 1863 and held its meetings in the Aurora Tavern.
Few of the neighbouring business operators were disappointed about seeing the building go down.
"It feels like I have lost a neighbour, but I am also happy that I will not be running my business next to a huge sex supermarket," said Mai Thai restaurant owner Bow Manoonpong, who has been running her business a few doors away for 21 years.
Adam Amar, who operates the 24-hour Ash Net internet cafe two shops away from the Palace, said he understood prostitution was legal in New Zealand but didn't like operating a business next to a brothel. He is hoping that what goes up next "will not be a bigger massage parlour".
Many did not want to be named or quoted for fear of being targeted.
"Of course none of us are happy about being next to a brothel, and what many of us worry is that the [type of] business also has links to Asian gangs."