Authorities have closed Wellington's Molesworth Street because of the risk of a building collapse.
Emergency services evacuated three homes and several buildings near the 10-storey high-rise, at 61 Molesworth Street.
Police cordoned off the street, and parts of nearby Hill Street, and diverted traffic.
Fire Service regional commander Brendan Nally said engineers doing a post-earthquake building check this afternoon found a major structural beam on the fifth floor had broken.
"A vertical beam in the building has been shorn, it looks somewhat like a broken bone in the leg, so it's fractured through.
"So it's a major supporting beam, it's up above the fifth floor, so it's the top four floors of the building [affected]."
Mr Nally called it a "clear structural failure". The engineers declared the building unsafe, he said.
The beam was by the lift shaft. The building was under renovation and was empty.
It was unclear what caused the damage or if the building would need to be demolished.
Mr Nally said the affected area would remain sealed off until decisions were made tomorrow about what to do.
Molesworth Street runs past Parliament and the National Library. It connects to the motorway and is a major thoroughfare for commuter traffic.
The evacuated buildings included New Zealand Rugby, St Pauls Cathedral, the Thai Embassy and the Red Cross.
New World Molesworth was outside the cordon and was open.
Nearly 50 buildings quake-damaged
Engineers have been inspecting nearly 50 earthquake-damaged buildings in the capital city, with some likely to be closed for weeks.
Yesterday people were asked to stay away from the CBD after several multi-storey buildings were damaged, and glass fell into streets, following the magnitude 7.5 quake centred near Hanmer.
Many of the damaged buildings were on reclaimed land on Wellington's waterfront, including the BNZ and nearby Statistics New Zealand building.
The 500 staff who work in the Statistics NZ building were told they could not return for several months after one corner was damaged in the earthquake, affecting two floors.
Statistics chief executive Liz McPherson posted a message on Facebook telling staff she would ask engineers how such a new building could suffer such damage.
A BNZ staffer said they were told it would be at least a week before they returned.
A Te Papa spokeswoman said engineers had inspected the building and found no structural issues. It was expected to reopen tomorrow.
Engineers were assessing the nearby Greater Wellington Regional Council offices.
Council chairman Chris Laidlaw said subsidence opened up gaps between the bottom of the walls and the floor.
He said it may be weeks, or longer, before all of the nearly 300 council staff returned.
Some CBD areas, including Featherston Street, remained cordoned off because of the risk of further glass and debris being dislodged by strong winds.
Commuter trains and bus services resumed today, but some had since been impacted by flooding.
Commuters arriving in Wellington this morning said carriages had about a quarter of the usual number of passengers.
Engineers had assessed the Westpac Stadium stadium and cleared it of any major structural damage.
The internal concourse and seating bowl would stay closed until repairs were completed in about two weeks.
The earthquake burst water pipes, cracked concrete, broke windows and caused liquefaction on the stadium ring road.
The stadium offices and the ground level of the public car park were open.