Mobile phone service for Vodafone, Spark and 2degrees customers in Auckland has been badly affected by today's power outage.
About 160 cell sites are in the affected region and all were running on battery back-up, which was now running out, the companies said.
The Vodafone mobile network had about 90 cell sites within the affected area that had no power, spokeswoman Michelle Baguley said. The sites required power to provide mobile coverage.
"We have some running on generators and some are partially operating," she said.
"We're looking to investigate adding more generators later on today from other parts of the North Island, but there will be areas of no mobile coverage inside that zone."
There was "no way of telling" how many customers were affected, Ms Baguley said.
As sites have their power restored, generators from those towers would be transferred to sites without generator back-up she said.
The Vodafone fixed line network is not affected, she said.
Spark spokesman Richard Llewellyn said the company had about 40 sites in the affected area.
"We don't know how many [customers] are directly affected in terms of home broadband or PSTN (Public Switched Telephone Network) services because without power, people can't turn on their modems or routers or electronic phone systems."
There was also some "degradation" at cell sites, Mr Llewellyn said.
"It's an ongoing situation."
Generators were being put onto high priority cell sites .2degrees corporate communications manager Charlene White said they had about 30 cell sites affected - mainly in the eastern area.
"We have battery reserves if we lose power to those sites, but given the duration of it, they are starting to run down."
They were also hoping to get generators to their sites.
"Our guys are trying to do the best they can, but obviously there will be patches where coverage is not possible."
For people in those areas, people would only be able to use land lines connected to phone wires, she said.