Power has been restored to most of Auckland, but many Northland customers are still without electricity. Photo / Brett Phibbs

Power has been restored to most of Auckland, but many Northland customers are still without electricity. Photo / Brett Phibbs

Removing the risk to the power supply to the top half of the North Island will take three or four years and cost $540 million, Transpower says.

Parts of Auckland, West Auckland, the North Shore and everything north of the Auckland Harbour Bridge were without power for several hours today when a container on a fork lift hit a high-voltage power line in Otahuhu.

Power was restored by 1pm, Transpower said.

The outage was caused after a forklift carrying a container knocked out the main source of power for the North Shore, parts of West Auckland and Northland.

Transpower chief executive Patrick Strange said the driver of the forklift was a "lucky" man.

"There would have been a very big explosion and I understand the tyres were all blown out," he told NZPA.

He said the North Auckland and Northland Project (NAaN) to reinforce the power supply through Auckland was still years away from completion.

"We are running a $540 million cable which will basically parallel this line. That will be completed in 2013 and put a transmission ring around Auckland."

The new 37km of cable would run from Pakuranga, in the eastern suburbs, under the road, through the Vector Tunnel, across the harbour bridge and in ducts on the northern motorway to Albany.

"This will remove the exposure to this single double-circuit line. It means that you could lose the cable or both circuits and we would have time to restore supply without affecting customers."

North Shore mayor Andrew Williams said the cut has given Transpower "a shock", causing people to be trapped in lifts and black-outs at traffic lights.

About 280,000 homes and businesses from West Auckland, the North Shore and Northland lost power this morning.

Mr Williams said 20,000 businesses were without power this morning and it caused "chaos" to traffic.

"There's a huge cost to people right now, twiddling their thumbs who can't do any work for an hour or so," Mr Williams said.

He said the cut highlighted the fragility of the New Zealand network.

"You can't be reliant on old over-head lines floating around West Auckland," Mr Williams said.