Police have allowed people to return to their homes in the tornado-hit area of West Auckland.
Hundreds of homes in Hobsonville and Whenuapai were damaged by yesterday's rare tornado and a number of them are beyond repair.
Auckland Council inspection teams have been going through the area to assess the safety of the structures.
Civil Defence said 240 homes in Whenuapai had been assessed and 270 homes in Hobsonville. Twenty-two properties had major damage and multiple properties had minor damage.
Inspectors said they believe "a relatively small number of buildings will be found to be structurally unsafe".
Prime Minister John Key said he expected a number of them would have to be demolished.
Visiting the area, which falls into his Helensville constituency, he said he had never seen anything like it in New Zealand before.
"It's far more significant than a very bad storm where you have a few trees down. It's something you might expect to see in the midwest of America, but not in Hobsonville.'
Earlier today, police named the three men who died in the tornado. They are: Tom Stowers, 42, of Massey; Brendon Johnson, 22, of Massey; and Keith Langford, 60, of Tuakau.
They were killed when large concrete slabs in the high school they were building fell on them.
Their employer, Hawkins Construction, said it was working to provide support for its families of workers and sub-contractors affected by the tornado.
"This has been a terrible tragedy for the families of those killed and injured, and everyone involved in the project is deeply shocked and saddened by what has happened," executive general manager Dan Ashby said.
Seven people are being treated in hospital for injuries received in the tornado.
The twister hit Hobsonville and Whenuapai around 12.30pm, as a system of heavy rain and strong winds passed over the North Island.
Other parts of Auckland were hit by localised flooding.
The same storm system also spawned a tornado over Rotorua yesterday afternoon.
Several vehicles were reported to have been thrown in the air in the lakeside community of Hamurana
Meanwhile, 570 households in the area are still without power. Vector said electricity might be restored to some of those this evening.
Many of the properties were badly damaged, a Vector spokesperson said. The company said power had been restored to 700 properties overnight Thursday but the low voltage part of the network would probably not be repaired until late Sunday.
The firm urged people not to use generators to power appliances in residential houses.
The affected area is under Civil Defence control and Vector said it would only make lines live where it was safe to do so.
A spokesperson for Whenuapai Air Base told ONE News this morning that more than 250 area residents went to the base yesterday. Eleven people were still there this morning, with the rest staying with friends and family.
Civil Defence said the Massey Leisure Centre recovery centre had closed for the day and would reopen at 10.00am on Saturday. The Victory Church Recovery Centre was closed and would not be reopened.