MetService warned of the possibility of severe thunderstorms from late Monday morning to late Tuesday morning. Surface flooding and slips were also possible due to heavy rain.
Surface flooding has been spotted on Whangārei Heads Road, Onerahi, and on Kaka Road in Morningside.
MetService meteorologist William Nepe said the weather would likely ease tomorrow and turn to showers rather than heavy rain.
Top Energy has recorded multiple power outages today in the Far North, in Coopers Beach, between Ōmāpere and Waimamaku, and an area north of Tangoake to Cape Reinga.
Waka Kotahi is warning motorists to expect heavier than usual holiday traffic due to the bad weather.
"If you are in Northland today and your plans are flexible, we suggest you delay travel," Waka Kotahi systems manager Kobus Du Toit said.
"If you must travel, drive to the conditions, slow down and maintain a greater following distance between your vehicle and the one in front.
"Stay alert for unreported hazards, such as surface flooding, slips, fallen trees and branches, and treat any downed powerlines as live."
Northland harbourmaster Jim Lyle said there had been a yacht run aground on the rocks at Tutukaka, and a launched beached at Opua, but no other reports of damage to boats yet.
Police put a warning on their Facebook page about the flooding on Northland's roads today, urging people to drive to the conditions.