Ongoing heavy rain is lashing the upper North Island, leaving hundreds of people are without power, and closing roads and schools.
State Highway 1 was closed at Mangamuka earlier this morning, and State Highway 10 has now been closed north of Kāeo, NZ Transport Agency Waka Kotahi (NZTA) says.
High vehicles and four-wheel-drives had reportedly been getting through at Kāeo but a number of smaller cars have come to grief and are now stuck in floodwaters.
The motorists are understood to have got out safely.
With the Far North District also closing Kaitāia-Awaroa Road, normally used as an alternative to the Mangamuka Gorge, that means there is now no road access to Kaitāia, Doubtless Bay, or the Aupouri Peninsula.
Water is still rising at Kāeo where rain is continuing and high tide is due around 2pm.
NZTA closed State Highway 1 near Mangamuka township early this morning due to flooding and at the slip-prone Mangamuka Gorge as a safety precaution.
Parts of Northland have already copped more than 200mm of rain - with the worst of the storm expected to come tonight.
MetService had issued a red heavy rain warning and the National Emergency Management Agency urged people to take that seriously.
"Red weather warnings are real and no joke, and I’m urging people not to put themselves in harm’s way, as your life safety is critical," NEMA director of civil defence emergency management John Price said.
"MetService only issues red warnings for the most extreme weather events. Heavy rain and severe winds can cause flooding and landslides, which can kill or cause serious harm.
"It’s tempting to go out and see what is happening, or go out on the water to experience the big waves - but please, trust your danger sense and don’t be foolish.
"If you get into trouble and need rescuing, you’ll be holding up emergency services who need to be looking after our most vulnerable."
Speaking early this afternoon, Emergency Management and Recovery Minister Mark Mitchell said there was the added complication of a second front to contend with.
He said everyone was in position, the mayors and the responders were all ready to take action if needed.
Mitchell urged people to have a grab bag ready, and for anyone concerned about flood waters, "make a move now while it's still daylight".
"We know we're going to get a substantial amount of rain, as well as wind overnight, but it's very hard to predict and it's not a perfect science.
"Localised storms which are rapid can create a lot of havoc so we're planning for the worst but hoping for the best."
Fire and Emergency New Zealand (Fenz) was earlier urging residents in storm-affected areas to be ready to evacuate if necessary.
A red weather warning remains in place for Northland east of Kaikohe from Doubtless Bay to Whangārei, with the worst of the downpours expected to hit on Thursday afternoon.
Marae in the region have been opened for those in need of support, and Fenz has deployed 19 specialist rescue personnel to Northland and Auckland.
As many as 5000 consumers were without power at one stage overnight, but that number had dropped to several hundred by 9am.
About 25 Northland schools have closed for the day.
MetService said the heaviest rain and largest volumes were likely to be in the upper North Island, from Northland to western Bay of Plenty.
Downpours, flooding, and slips were also possible on Thursday and Friday.
Fenz assistant national commander Ken Cooper warned residents in upper parts of Northland to be ready in case the situation deteriorated.
"For that upper part of Northland, the intelligence we've got is there's a large amount of rainfall over a very short period of time. I would certainly advise people to be prepared, if they're in low lying areas or near rivers, be prepared to move."
Cooper said anyone concerned about a risk to life or property should call 111.
Northland Civil Defence expected the worst of the rain to hit the northeast coast on Thursday night.
In a post on social media, it warned residents not to go into flood water, to avoid unnecessary travel, and to be aware of slips.
"Leave immediately if you notice cracks in the ground, leaning trees or power poles, unusual sounds, or sudden changes in streams," it said.
Whangārei Mayor Ken Couper told Morning Report it had been a wild night with strong winds and consistent but not heavy rain in Waipu, just south of Whangārei.
It was too early to say whether the situation was different up the coast, he said.
"We are concerned because of the weather event that we had in January and the fact we're still recovering from that."
Power was out last night in Oakura, which was affected in the last storm, he said.
All of the 200 slips except for a massive one had been cleared after the January event, he said.
"But there's a number of sites where we were hoping for more time to pass to get things more stable before rain came."
