Power is now back on after outages in many parts of the North Island caused by a failure at the large Haywards substation north of Wellington.

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There were reports of a loud bang at the substation, in the Hutt Valley, before the outages. The substation is operated by national grid provider Transpower.

Transpower said all power was restored shortly before 1pm.

It was understood 190,000 customers in Auckland were cut off, however it's not known the total number of people affected throughout the North Island.

Transpower chief executive Patrick Strange said the company had been testing its inter-island DC link when it malfunctioned.

"What we do as one of our tests is simulate a line fault. We cause one of the lines to trip to make sure the protection will work.

"We were running a lot of power from the South Island to the North Island ... Something has happened with a malfunction."

Between a quarter and a fifth of electricity customers in the North Island experienced outages

"It's affecting a lot of people and we're concentrating on getting them back on and then we'll find out what happened and fix it. But that's why we do these tests," Strange said.

Auckland power company Vector said it was aware of multiple outages across the city, and to the north in Warkworth.

There were reports of traffic lights being out in Hamilton.

Since power was restored residents near Haywards reported sounds of further explosions, however Senior Sergeant Marc Clausen said this was likely from the Police Special Tactics Group which was training with explosives in Petone.