A large number of properties in west Auckland have been cut from power after a substation overload.
A Vector power spokesman said the exact number of houses on and off the grid was constantly changing as they tried to restore power to affected areas.
"We expect to have all customers affected by this new outage back online tonight."
Crews have reported they are making good progress, with a number of feeders restored in the Northern network where the worst damage was experienced, he says.
Vector apologised for the inconvenience for customers.
Glen Eden resident Randal Meikle told NZME his home lost power at 6.10pm this evening.
"We were plunged into darkness.
"I had to drive towards the city at 7.15pm and could a large portion of Glen Eden, New Lynn and Green Bay also in darkness."