A severe weather watch is in place for the top of the North Island with a heavy rain system from the north Tasman Sea reaching land this morning.
Northland was being buffeted by gusts of up to 90km/h and rain had started falling as the sub-tropical low moved south towards Auckland.
The New Zealand Transport Agency warned motorists to take care on Auckland and Northland roads.
"Because we've had so much sunshine in our regions, expect road surfaces to be slippery with the rain," NZTA regional assets manager Steve Mutton said.
"All summer long, dust, dirt, oil and other debris builds up on the surface and when it rains those ingredients combine and get stirred up to make driving conditions slippery and dangerous."
He said road conditions would be worst in the first few hours of rain and "light rain is more of a challenge than a heavy downpour".
Drivers should reduce speeds to 10km/h below the legal limit and increase distances from other cars, Mr Mutton said.
Meanwhile, falls of up to 200mm are predicted for Fiordland as a significant weather system sits over the bottom of the country.
Wet weather this week is expected to break the drought for some areas of the North Island where farmers have endured one of the driest summers in more than 50 years.
"So far it's started raining in Northland ... and that's the main rainfall we're waiting to spread further south," MetService meteorologist Brooke Lockhart said this morning.
She said there has already been "patchy rain" in the Bay of Plenty and Auckland was likely to see falls later today.
"It will start getting heavier in Northland this morning and then the rainfall will slowly spread further south.
"Auckland's looking to get a bit of rain early this morning and becoming heavier later in the day.
"All of the upper North Island is expected to get some sort of rain throughout the day. Overnight everywhere south of Taranaki [is expected to receive rain]."
Ms Lockhart said forecasters were still working on rainfall predictions for the North Island.
A new severe weather watch will be released later this morning.
"At present, it appears that the Coromandel Peninsula and Bay of Plenty are most likely to experience warning amounts of rain, but eastern areas of Northland and north Auckland may also come close to warning criteria. If necessary, warnings will be issued closer to the event."
Electricity provider Vector said customers should be prepared for the possibility of outages in the expected stormy weather.
It had put its field crews on alert.
Vector reminded people that in the event of an outage that they should stay clear of fallen power lines or damaged electrical equipment, watch out for falling tree branches, keep a torch, spare batteries and a phone handy and to ensure all outdoor equipment and furniture was secured.
"The public should be confident that Vector will work hard to restore power as quickly as possible at all times. The safety of the public and Vector's contracting field staff is of primary importance," it said.