Transpower has issued a grid emergency notice for the North Island.
The notice, issued shortly before 4.30pm this afternoon, says there is insufficient generation and transmission capacity.
The emergency notice is in place from 5pm this evening until 7.30pm.
It also refers to the unavailability of HVDC Pole 2, which forms part of the Cook Strait cable connecting the two islands.
Chief executive Alison Andrew said in a statement that a conductor (wire) had fallen from a tower around State Highway 7 in the Weka Pass area, Waikari in North Canterbury. There is no indication anyone had been harmed, she said.
She said that the market was currently stable however, if there was a major plant failure, there was a risk of not being able to meet demand through the evening.
Power generators are being asked to increase supply.
"We have asked electricity distribution businesses (lines companies) in the North Island to reduce all controllable demand (such as hot water). If this does not achieve the reduction we require, we will issue a further notice setting a further target for demand reduction."
Power company Genesis says it has ramped up generation at its Huntly power station in response to the Transpower emergency notice.
It says it has put the Huntly station into overload mode and will run one unit beyond its planned shutdown time this evening.
The company says it's also started firing up two other units, which won't come on to line until about 3 am, but willl be available to cope with peak demand in the morning and through the day.
Crews are also at the site in North Canterbury to do the repair work.
The announcement comes after thousands of homes in the North Island abruptly lost power on 9 August.
Transpower, the state-owned enterprise that owns and operates the national grid, cut off the power because it said there was not enough to supply the country, on one of the coldest nights of the year.
Energy Minister Megan Woods immediately demanded answers from officials, lines companies and power generators.
She blamed Genesis, saying it had decided not to turn on its third generator at Huntly. She also said there had been enough electricity but commercial decisions were behind the decision.
Genesis said it was being made the scapegoat for the outages.
It was also forced to apologise to power users in Hawke's Bay and Tairāwhiti for confusing messages it posted on Facebook yesterday.
They related to power being lost for about half an hour in the two regions.