Flooding is threatening homes in Porirua, and has closed roads, while MetService warns of widespread storms, including the possibility of tornadoes in some areas.
At least three homes in Porirua have flooded as bad weather continues to batter the area.
Porirua mayor Anita Baker said floodwaters were rising around some houses, and the problem was expected to get worse with ongoing bursts of torrential rain, and stormy weather forecast to continue into tomorrow.
"It's gone into their garages - I've got a lady who's keeping in constant contact - it's already in their garages, but their house is a little bit higher, but it's surrounding their houses and their heat pumps and things like that, so it can't be far away."
Porirua City Council has activated its Emergency Response Centre, and Civil Defence Controller Sam Bishop said there is flooding across much of the city.
"For the most part it's through gardens and some garages, however we are still assessing the full extent of this event.
"We've had many other reports of flood water on people's properties."
Bishop said if people decide they need to evacuate they are asked to let the council know, on 04-237 5089, or emergency services if there is significant risk.
He said there are also a number of slips on the state highways in and out of the area, and on smaller roads as well, and people should avoid travel unless necessary.
Bishop said they are starting planning for an evacuation centre, in case it is needed.
The situation is changing quickly, and the council was keeping an eye on things with high tide expected at about 8pm.
Fire and Emergency NZ was responding to multiple calls for help in the north of Porirua because of flooding, and an Emergency Operations Centre had been activated.
Baker said homes in Mana and Plimmerton were at risk, and firefighters were operating a pump to try to clear the water.
Baker asked people to stay off the roads if possible, and if they were driving called them to watch for sewer manhole covers which can pop out with flooding. Anyone who needed help should contact the council or Fire and Emergency for assistance.
Flooding at Plimmerton Roundabout on SH59, north of Wellington, at 1pm Sunday. Photo: Supplied/ Waka Kotahi
Flooding along the length of Mana Esplanade, in the north of Porirua, had covered both lanes of State Highway 59 (which was SH1 until recently), with water up to cars' bumpers blocking it to traffic. By 2pm, Waka Kotahi said the road had reopened, but still had problems with surface flooding. They asked motorists to take extra care.
The Paremata New World supermarket on Mana Esplanade was surrounded by deep water, Baker said.
The water had begun coming into the store, and staff had put sandbags around the doors.
Flooding on Mana Esplanade in Porirua has covered the road and surrounded some buildings. Photo: Supplied
Nearby, part of State Highway 58 was closed, around the Pāuatahanui Inlet due to a slip. Several kilometres inland in Porirua East, Warspite Avenue was also underwater.
"It's like a lake has appeared. The rain has been incredibly heavy," an RNZ reporter said, when they were forced to turn back from Warspite Avenue.
Waka Kotahi said the weather was affecting traffic on highways throughout the Wellington region, and asked motorists to be vigilant for hazards.
Wellington resident Josie Nixon was travelling through Porirua today, and said parts of Whitby and Paremata were badly affected
"We had to drive through really deeply flooded areas. At the bottom of Postgate Drive ... half of the playground was flooded under - there was a little bridge that the kids usually walk across - you could only see the top of the bridge, it was so deep.
"Manholes were popping out and water was pushing out from them."
Fire and Emergency NZ assistant commander Martin Wilby said six emergency response vehicles were working on storm-related calls in Porirua, including pumping out water to protect homes.
A command unit was operating from the Plimmerton Fire Station along with the Emergency Operations Centre in Porirua.
MetService meteorologist Stephen Glassey said an unsettled, unstable north-west flow was expected to bring severe thunderstorms, heavy rain, and strong winds. The northern and western parts of the country would be worst affected.
There was the possibility of small tornadoes along the west of both the North and South Islands today.
"We've got a number of warnings and watches out for severe gales, heavy rain, and in fact snow in the south.
"As well as that we've got road snow warnings out ... and quite large swells as well, especially in the west of the country."
Glassey said it was still snowing in parts of Central Otago and the Canterbury High Country, but it was not falling to such low levels as it did yesterday.
Photo: RNZ / Rebekah Parsons-King
Horowhenua water treatment back, storms loom
Water supply issues in Horowhenua due to treatment plant problems had eased, but concerns remained about whether continuing heavy rain could create further interruptions.
An urgent call went out yesterday asking residents in the area, north of Wellington, to conserve water to avoid overloading the water treatment plant during heavy rain and winds.
Power was cut to many properties when lightning strikes hit power lines and transformers, and warnings were placed on roads due to flooding.
Horowhenua mayor Bernie Wanden said the treatment plant was back up and running, but there were concerns over whether the plant would be affected by ongoing stormy weather.
A severe thunderstorm, strong wind and heavy rain watch was in place for Horowhenua for the rest of Sunday and for Monday.