Demolition work has got under way in central Christchurch to remove buildings devastated by Saturday's earthquake.
Heavy machinery began pulling down condemned buildings to make the city safer earlier on Monday afternoon.
The demolition work will allow officials to reduce the areas cordoned off in the central business district.
The work was also taking place in suburban Christchurch. A block of shops was being torn down after a sharp aftershock before 1pm caused part of the building to collapse further.
About the same time, a severely damaged building collapsed in the Christchurch suburb of St Albans. People near the building, on the corner of Cranford and Westminster streets, reportedly had a narrow escape.
Lines company Orion says people are putting themselves at risk by demolishing quake-damaged houses before they have been completely removed from the power network.
Chief executive Roger Sutton says several such instances occurred on Monday as people not experienced in demolition procedures got involved in the work.
Mr Sutton says people needed to ring Orion and state clearly they wanted the building isolated for demolition.
Builders say they have the numbers for reconstruction work because of the recession.
The Registered Master Builders Federation says it has been in contact with tradespeople throughout the country on Monday. Chief executive Warwick Quinn says builders are ready for the challenge ahead.
Four years ago it would have been difficult to respond to the disaster, he said, but the industry is at only about half of its capacity because of the recession.
One of the South Island's biggest home builders says builders will probably have to be called in from other cities to help rebuild parts of Christchurch.
Christchurch-based Stonewood Homes managing director Brent Mettrick says many homes will have to be rebuilt because their foundations have been damaged.
Christchurch Medical Officer of Health Ramon Pink says gastroenteritis is inevitable in the city if people do not take precautions.
Dr Pink says damage to water and sewerage systems raises the risk of gastroenteritis, and he has had unconfirmed reports of 12 cases since the quake.