Gale-force winds of up to 150km/h and giant waves are pounding the capital, roads are covered in debris and flying trampolines made the outdoors hazardous.
About 12mm of rain has fallen in the capital so far, but more than 50mm is forecast for the rest of the day.
Most of the heavy rain is due to begin falling about 4pm, and nearly 37mm is expected between 4pm and midnight.
Eyes are on the rising Hutt River, which is due to reach high tide at 8.15pm in the middle of the deluge.
In the Hutt Valley, surface flooding is already starting, as large puddles stretch across fields and golf courses.
The speed limit on the coastal Urban Motorway has been dropped from 100km/h to 60km/h as motorists drive through crashing waves.
NZTA regional transport systems manager Mark Owen is urging people to take extreme care while driving, and to consider delaying travel if possible.
The Hutt Valley, Kapiti, Horowhenua and the Manawatu regions may get more heavy rainfall and high winds over at least the next 24 hours.
Road crews are on standby throughout the region, Owen said.
Wellington Zoo's animals are hunkering down as the city around them is battered by a vicious storm.
The zoo posted a picture this morning of Meluin the Tasmania devil snug and warm in a cosy blue blanket.
"On days like today we like to give our animals plenty of options for warmth," the zoo tweeted.
Rocks and trees have come down near the intersection of Hutt and Onslow Rds. Several other slips have also come down along Hutt Rd and a tree has blocked half of Waru St in Khandallah.
Train services between Upper Hutt and Wellington are running again after a fallen tree blocked the tracks, stranding the 10am service to Upper Hutt at Ava station.
Trampolines have been found in the middle of roads in Whitby and in Tawa. In Ngauranga, near the Newlands off-ramp, a 44-gallon plastic barrel is reportedly flying around the motorway.
Further north, cones have been blown over part of the southbound lane on State Highway 1 near Johnsonville.
More than 1000 people are waking up without power in the Wellington region this morning. In Newlands, Johnsonville and Paparangi, 480 people are affected and power isn't expected to be restored for two hours. In Upper Hutt, 580 people are without power in Whitemans Valley, Blue Mountains, Mangaroa, Maidstone and Wallaceville.
There are also outages around Paekakariki, Judgeford and Pauatahanui.
All train services are running, but there are delays on the Hutt Valley, Melling and Wairarapa lines. MetLink says further disruption is possible later in the day.
All harbour ferry sailings have again been cancelled in Wellington because of the weather and strong winds have delayed and cancelled flights to and from Wellington Airport.
All regional flights are suspended and the airport asks that travellers check with their airline or the Wellington Airport website about flight times.
NZTA is urging people to check the latest weather conditions before jumping in the car today, and consider alternative transport, as it warns of disruption on every major highway.
It says severe rain is forecast in Kapiti, which could cause flooding on State Highway 1 at the Plimmerton roundabout and on nearby Grays Rd.
The Paekakariki Hill Rd may have wind gusts of 140km/h, which could cause slips.
It's snowing on the Rimutaka Hill Rd, which is also likely to be hit by gusting winds, driving rain and sleet today.
Flooding is also possible on roads in Porirua and Churton Park.
