MetService meteorologist Arno Dyason said the thick fog was likely to stick around until midday.
"It is quite extensive, from Northland down to Whanganui, covering much of the central North Island too."
It formed in the wake of yesterday's slow-moving front, which left behind some warm and humid air, and a clear night sky.
"Those conditions really gave a good opportunity for fog to form."
Once it cleared most parts of the North Island were in for a fine winter's day.
The main weather story of the day was on the South Island's West Coast.
MetService had issued a severe rain watch to this evening about south Westland and Buller.
Tomorrow this front would make its way north, becoming stationary and spreading rain over the northeast of the North Island.
It was forecast to hover over Bay of Plenty and Gisborne through the weekend, before moving east on Monday.
There was still some uncertainty about exactly where would see the heaviest rain, Dyason said.
"But the focus will be on eastern Coromandel, Bay of Plenty and Gisborne, but most eastern areas will get some rain, even in the South Island.
"All parts exposed to the north, including Northland and Auckland, should get some rain over the weekend."
A weak ridge of high pressure would cover the South Island, bringing mostly fine weather from Friday through Sunday.
Northerlies would strengthen over the South Island on Monday ahead of a front approaching from the Tasman Sea.
Today's weather
Whangārei
Mainly fine with morning fog. High cloud increasing evening. Northwesterly developing morning. 17C high, 9C overnight.
Auckland
Fine, but areas of morning fog. High cloud increasing evening. Northwesterly. 16C high, 10C overnight.
Hamilton
Cloudy with morning fog, but also some afternoon fine spells. Northwest developing morning. 15C high, 7C overnight.
Tauranga
Partly cloudy. Northwest developing evening. 15C high, 9C overnight.
New Plymouth A few showers, turning to rain in the evening, possibly heavy. Northwesterlies, strengthening evening. 14C high, 8C overnight.
Napier Fine, some high cloud from afternoon. Northerlies. 18C high, 7C overnight.
Wellington Cloud increasing with the chance of a shower. Rain at night. Brisk northerlies. 14C high, 10C overnight.
Nelson Rain developing late morning, chance heavy. Northerlies. 13C high, 5C overnight.
Christchurch Cloud increasing, and a few spots of rain from afternoon. Light winds. 15C high, 4C overnight.
Dunedin Becoming cloudy, occasional rain afternoon, more persistent evening. Northeasterlies. 12C high, 8C overnight.
