Power has been restored to almost 90,000 customers across Northland and north of Breams Bay, power companies say.

Photo: Supplied / Top Energy

Northpower this morning said most of the 60,000 properties it supplies - about 48,000 - were affected, and Top Energy in the Far North said 32,000 of its customers were without power.

Just after 12pm, both Northpower and Top Energy said power had been returned to their network.

Transpower said one high-voltage circuit into Northland was down for maintenance when the other tripped about 9.30am.

It said it was not known what caused the circuit to trip.

Homes would be without electricity for at least two hours, it said.

RNZ's radio transmitters from Whangarei north were also without power.

Refining New Zealand said it had also been affected by the outage. Marsden Point Oil Refinery had to completely power down for safety reasons, and is now restarting operations.

Power was also fully restored to the Northpower Network, which covers 60,000 properties including Kaipara and Whangārei.

In a statement, it said: "The plant has been safely shut down with no injuries and no identified damage to processing units, and with safety critical systems being maintained by emergency back up power.

"During these processes flaring will occur as a mechanism to vent refining gasses safely as the processing units are depressurised."

Northpower said crews were working hard to reconnect people, with priority to Whangarei hospital and emergency services.

The Whangarei District Council said it was not clear when power would be restored so was urging people to conserve water and avoid flushing toilets.

The council's water supply and sewer systems require power.

Traffic lights are also out in the region.

The council's infrastructure general manager Simon Weston said the less stress placed on those systems, the easier recovery would be when power is restored.