At least one person is dead and more than 30 are trapped in the Pyne Gould Corp (PGC) business building in central Christchurch.
The four-storey building collapsed in the magnitude 6.3 quake, which struck at 12.51pm today.
Labour leader Phil Goff said from outside the building at least 30 people were trapped inside.
"I know that there is one person who is fatally injured but they still cannot extract them," he told Radio New Zealand.
"This is a 1960s building, it's not one of the old brick buildings, and I was told by the police that there may be a dozen buildings like this around Christchurch - dozens of people trapped and the fatalities may be much higher than we imagined.
"We hope not but we know that at least there will be a significant number of casualties. I'm told that bodies have been recovered from buildings.
"It's a real tragedy for Christchurch."
Police and fire service personnel, as well as construction workers, were frantically working to free those trapped inside the building.
"They know some of the people are trapped but otherwise relatively uninjured but they don't know the situation of other people," Mr Goff said.
PGG said in a statement to the stock exchange it was concerned for its staff.
The building houses PGC, Perpetual Trust and Marac.
PGC managing director Jeff Greenslade said all efforts were being made to contact staff in the area.
"Our primary focus is on accounting for all staff.
"We are endeavouring to make contact with all of our staff in Christchurch.That is our over-riding priority."
PGC executives were on their way to Christchurch to offer any assistance they could, Mr Greenslade said.
Staff had been told to attend to their own needs first.