Transpower was unsure when normal operations would resume, but was aiming to have power restored by 7pm, chief executive Patrick Strange told Radio New Zealand.
Auckland Mayor John Banks said the power cuts being experienced across Auckland were the result of "reckless management" by Transpower.
Power was cut between Huntly and Whangarei after an outage at a transmission line south of Auckland around 3pm today.
High-voltage power lines fell onto trees near Tamihere, about 9km southeast of Hamilton, causing a scrub fire and power outage at around 3pm, a spokesperson for northern fire communications said.
Power to Auckland has been cut to about 5000 homes by lines company Northpower at the request of Transpower.
Outages have been reported in the Auckland suburbs of Epsom, Mt Eden, Remuera, Mt Wellington and on the North Shore in Takapuna, Glenfield, Cheltenham, Northcote and Henderson.
On the social networking site Twitter Aleisha Moore writes: "The power is out here where i live near Ramarama."
Treewheeler writes at 6.25pm: "Can't believe that power isn't back on yet in botany! Very slack."
Another nzherald.co.nz reader writes: "The power went out here around 4.30pm and is still off at 6.22pm."
Cameron Brewer from the Newmarket Buisness Association writes: "Half of Newmarket businesses lost power around 4.45pm this evening. Fortunately it's at the end of the day, but it does nothing to inspire confidence about the security of electricity supply into Auckland."
nzherald.co.nz readers have also reported traffic gridlock in the Auckland suburb of Ponsonby with traffic backed-up all the way down College Hill and along Ponsonby Road.
Mr Banks said it was "ridiculous" that power cuts of this magnitude could result from a fire on private land.
"How could that possibly close down so much of Auckland?" he asked.
Mr Banks said up to 10 per cent of the city had been "shut down" by the power cuts, and traffic was gridlocked.
"[The power cuts] are a result of under-investment, callous disregard by Government and reckless management by Transpower," Mr Banks said.
He said Transpower executives "on their huge salaries" would be "held to account" for today's power cuts.
A police northern communications spokeswoman said Auckland, Counties Manukau and the North Shore were affected.
Senior Sergeant Chris Money of Auckland police said large areas of Manukau and Auckland City were out of power.
Mr Money said many traffic lights were not working.
"We've got traffic cars all over Auckland trying to deal with the traffic lights," he said. "It's particularly busy for us because of it."
He said the police were fielding large numbers of calls from the public related to the power outage.
- NZ HERALD STAFF
