National grid operator Transpower says it is disappointed a botched routine test led to a tenth of the North Island losing electricity on Tuesday.
Traffic lights were affected, shops had to stop trading and students sitting NCEA exams were plunged into darkness.
Transpower says one out of every 10 customers in the North Island lost power, with 190,000 cut off in Auckland alone for about two hours. Vector said the Auckland network was brought back online at 1pm.
Electricity was also cut in parts of Wellington, and lines company Powerco said 25,000 homes and businesses in Tauranga, Taranaki, Whanganui, Rangitikei, Manawatu and the Wairarapa lost power.
The problem happened during routine testing of the HVDC link, a $700 million line between the North and South islands and known as the Cook Strait cable, due to be completed in December this year.
Transpower said it was simulating a fault by deliberately dropping a piece of piano wire on high voltage lines - a test it has done before to mimic a lightning strike.
Chief executive Patrick Strange said the outage is a big disappointment and Transpower is working on how to stop it from occurring again. He said it's only the second time this has happened in a decade.
As a last resort, Transpower tripped some customers to ensure that the system stayed stable and then restored them over the next hour in a protection sequence, he said.
Transpower said that by 1pm, all customers had their power restored. Dr Strange said while it was not so difficult for residential customers, he acknowledged that the cuts were a hit for manufacturers and retailers and apologised for the inconvenience.
Dinesh Sharma, a retailer at St Lukes mall in the Auckland suburb of Sandringham, said the outage happened just before the lunchtime rush and store owners were "losing business big time". Customers in the mall were urged to leave for safety reasons.
The power cut affected students sitting NCEA exams. The country's second largest school, Mt Albert Grammar, said students were plunged into darkness during accounting, biology and scholarship English exams, but staff were able to reassure them and move them to areas with more natural light. The Qualifications Authority has been notified.