People in Hawke's Bay and Gisborne are being warned it could be "days to weeks" before power is fully restored, due to the impact of Cyclone Gabrielle.
The regions suffered widespread flooding and damage from high winds as the North Island was battered by the weather system.
Amid issues with power cuts, Transpower has issued a grid emergency after flooding to the Redclyffe substation in Hawke's Bay.
It means the Unison and Eastland networks that distribute electricity to Gisborne and Hawke's Bay are down.
Transpower currently can't communicate with the substation, but believes it is under water.
"While we cannot currently access the site, we have a helicopter on standby for further investigation when possible," a spokesperson said.
"Until we are able to gain access to the substation we will not have a clear picture of how long the region will be without power, but we are advising that the community should be prepared to be without power for days to weeks, rather than hours.
"We are working closely with Unison and Eastland to understand the situation and develop a restoration plan to ensure power is returned as quickly as possible. All options are being considered, but the safety of our work crews and the community is paramount. We will not take actions to bring power back before it is safe for them to do so."
Transpower said there was a brief outage in Taranaki overnight, and the grid "held up" in other parts of the country.
Tens of thousands of households remain without power across the upper North Island.
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