Much of the Taupō district is without power.
Unison posted on their Facebook page that a Transpower fault at the grid exit point of Wairakei Power Station was the cause of the outage.
Unison said they were working with Transpower to restore power soon.
Transpower posted an apology to their Facebook page for the outage, and said they estimated power to be restored by early Thursday evening.
Taupō District Council posted to Facebook that the town's customer service centre was closed but locals could still call, although there may be a delay.
Some Taupō residents tweeted about the extent of the outage.