Cyclone Gabrielle is already bringing strong winds and heavy rain to parts of the North Island.
Most of the North Island is covered by some kind of Severe Weather Watch or Warning either for wind, rain or both.
Red heavy rain warnings have been issued for Northland, Auckland, the Coromandel and the northern parts of Gisborne Tairāwhiti.
Red strong wind warnings have been issued for Northland, Auckland and the Coromandel.
RNZ will continue live coverage from 5am Monday morning and update any major developments overnight.
* Read more: Cyclone Gabrielle's impact in numbers
Thousands are without power, while some people in Coromandel and Gisborne are already starting to evacuate, and Auckland Council is closing all non-essential services Monday and Tuesday.
Power company Vector said there were widespread power outages, especially in exposed areas across north Auckland.
It said about 15,000 customers were without power at 7pm.
"This number will change constantly as we restore homes and businesses but, with the weather due to worsen, we expect further outages overnight and tomorrow."
Northpower has also been working to restore power in the Whāngarei area, writing on Facebook that "cyclone winds have caused major damage to the network, much of it from trees falling through lines."
Auckland Council has said it will close all non-essential services across the region on Monday and Tuesday, including libraries, community centres, early childhood education centres, and active recreation centres.
The Auckland Harbour Bridge has been closed due to high winds and will remain closed overnight.
All Auckland train services have also been cancelled tonight, and KiwiRail has closed operations on the top half of the North Island until at least 3pm tomorrow.
Meanwhile, Coromandel Civil Defence controller Garry Towler says six evacuation centres will be open 24-7 while another 12 to 15 community-led centres are on standby to open if required.
* Read more: Cyclone Gabrielle in photos
An emergency alert text was sent to all people on the peninsula this afternoon.
Towler says most people appear to be heeding the warning, as traffic leaving the region is heavy. He's advising people to go to family or friends.
Earlier, Northland declared a precautionary state of emergency for an initial period of seven days.
Speaking at today's official update, MetService meteorologist Georgina Griffiths said even with significant wind gusts in Northland already being reported, the weather today was just the start.
She said they had not seen pressure this low in 40 years.
"This is a serious event for New Zealand."
Follow the latest updates on RNZ's live blog below: