Powerco contractors are working to restore power to around 29,000 customers in the Coromandel, Thames Valley and Hauraki Plains areas.
Power was cut just after 10pm following a substantial grid outage involving Transpower’s electricity network, Powerco Acting Network Operations Manager Dean Stevenson said.
Transpower operates the national grid.
Investigations are underway but it is believed the outage then cut power to 18 Powerco sub stations.
It is unknown at this stage what time power will be re-connected, Mr Stevenson said.