"During this morning's switching - making the site safe - as part of routine maintenance, incorrect earthing was applied. This resulted in an arc of electricity similar to a flash of lightning – and the loud bang and smoke seen.
"There was no fire, no explosion of equipment. Our protection systems immediately cut the power supply."
It's believed to be a result of "human error".
The Hutt Valley, Wairarapa and parts of Wellington experienced a widespread power outage after the explosion at the Haywards substation. Smoke could be seen rising from the facility.
Trains on the Hutt line were halted and residents were asked to conserve water and to not flush the toilet.
The power outage began just before 8.20am and Transpower said shortly before 10am that power restoration was now "well under way". The work was completed by 10.40am, Transpower said.
Trains are not up and running yet, despite power being restored.
A Metlink spokesman said there were a number of checks that needed to be carried out after an outage before it was safe to resume operating - but it was hoped they would be back in service by the afternoon peak.
Hutt Valley bus replacements are departing Wellington every 20 minutes.
Wellington Water has confirmed power has been restored to the affected areas of the water network and normal operations had resumed.
"On-site 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," it said in a statement.
"Residents had been advised to limit water consumption and toilet flushing in order to reduce demand on the network as a precaution. This advisory has now been rescinded, other than in South Wairarapa, where a request to conserve water consumption remains in place."
Wellington Water is now investigating the wider network to verify everything is operating as normal.
"Wellington Water reminds all residents of the importance of having their drinking water storage and waste disposal plans in place in case of a much more significant interruption of services following a major event such as an earthquake."
More than 94,000 houses lost electricity
Wellington Electricity spokesman Dave Mulligan said an issue at a Transpower substation cut power to 63,000 properties across Lower Hutt, Upper Hutt and parts of the Wairarapa.
Another 31,000 customers were also affected in the Wairarapa, Powerco said.
Transpower had earlier said it could take an hour for power to be restored "although it might take a little longer for Wellington Electricity and Powerco to reconnect everyone".
Hutt Valley and Wairarapa Outage Update # 1
Kia ora Hutt Valley and Wairarapa. Crews are on site at Haywards and we now...
Posted by Transpower NZ on Wednesday, 11 March 2020
Wellington Water said the outage was affecting drinking water and wastewater pump stations and treatment plants across the wider Wellington region and Wairarapa.
"We are asking people to reduce their water use and to avoid flushing the toilet until the power is returned."
Traffic lights are not working on State Highway 2.
All trains on the Hutt line have been suspended due to a "citywide blackout", Metlink said.
Services departing from Wellington railway station are also affected.
"Some services are being held at stations until they can be given verbal consent from Train Control to pass the signals that are having issues," Metlink said.
"Maintainers are en route to assess the issue and isolate the points systems along the Hutt Valley Line.
"Services departing from Wellington to Melling, Upper Hutt and Masterton are suspended until further notice.
"We are attempting to source bus replacements to run on the Hutt Valley Line between Wellington and Upper Hutt."
A Fire and Emergency spokeswoman said a member of the public reported smoke coming from the substation in Lower Hutt just before 8.20am.
It was later confirmed there had been a small explosion in a switchyard, which had cut power across Hutt Valley, Wairarapa and even parts of Wellington, she said.
There were no reports of injuries.
A Capital and Coast DHB spokesman said Wellington Regional Hospital still had power, and all services were running as normal.
