People cruising the streets of Whangarei and Kaipara on Monday night were suddenly plunged into darkness with the outage of almost all streetlights across both districts.

The outage occurred mainly between 6pm and 6.20pm, though some went out again briefly about an hour later, said Northpower spokesman Steve Macmillan.

This followed a problem with a non-Northpower North Island generator, but Mr Macmillan said the streetlight outage should not have occurred and was a Northpower issue.

"Our engineers are looking into it ... We want to get on top of it to make sure it doesn't happen again," he said.

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Mr Macmillan said the generator fault happened just before 6pm, which automatically reduced the hot water and other controlled electricity loads in Northland - the correct process for such an event. However, a glitch in the automatic shutdown system meant the street lights also turned off.

Northpower worked urgently to get the streetlights back on. "What happened nationally was exactly what was supposed to happen. This was down to an engineering glitch at the Northpower end," Mr Macmillan said.

He was unsure of the exact number but said most streetlights across Whangarei and Kaipara were off.