The power went out just after 5.20pm and the outage covers the entire southern part of the district, including the Bay of Islands, Kaikohe, South Hokianga and Whangaroa.
It's the same area that was affected by a huge outage last Friday.
In that case the cause was a tree falling on a 33-kilovolt line near Mount Pokaka, south of Kerikeri, but it was not clear why that caused the Kaikohe substation to shut down, cutting power to more than half the district.
The northern part of the district, including Doubtless Bay, Kaitaia, the Aupouri Peninsula and North Hokianga, still has power, in an apparent re-run of last week's outage.
According to Top Energy's outage centre, the fault is on a sub-transmission feeder but the cause is not yet known.
