An explosion at a Transpower substation left 63,000 households in the Hutt Valley and Wairarapa without power.
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A loud bang was heard about 8.20am and smoke was visible above the substation. Fire and Emergency said crews attended but there was no fire to extinguish.
The outage caused train delays, business disruption and affecting water supplies.
Transpower chief executive Alison Andrew said the blackout was caused by a switching error during routine maintenance.
"We'll be doing a detailed investigation to find out if there's any damage to equipment and what we can learn from the incident."
No-one was hurt in the explosion and power was restored shortly before 10am.
Commuter trains on the Hutt Valley and Melling lines were suspended due to glitches in the signalling and points system following the blackout.
Services started to return to normal from midday, when Metlink had clearance to run a service on the Hutt Valley line.
Some trains were being replaced with buses and regular buses were accepting train tickets, the operator said.
The power outage also affected the bus real-time tracking systems.
And traffic signals in Lower Hutt have been affected as well.
The Transport Agency said drivers should slow down when approaching affected intersections and apply the give way rules.
The electricity cuts affected businesses in the Hutt Valley.
Andrew Stone from Moera Pharmacy in Lower Hutt said he had to turn away customers.
"We couldn't really do anything while the power was off. Our phone's on VOIP, so it's on internet as well.
"We had to keep our fridge closed just to make sure the fridge stock is kept at the right temperature.
"It was lucky that there wasn't any medical emergency."
Wellington Water was checking all systems are operating as normal following the major power outage.
A spokesperson said says generators at critical pump stations and treatment plants were rapidly brought online, and mobile generators were being deployed to other stations when mains power was restored.
Supplies were back to normal except in South Wairarapa where water restrictions were already in place, the company said.