Coastal residents in Porirua and Kāpiti are being warned to prepare sandbags and shift their cars ahead of a heavy swell warning from midnight.
Wellington Region Emergency Management Office manager Jeremy Holmes said residents could expect slips, scouring flooding and dumps of driftwood, seaweed, sand and gravel.
People in Titahi Bay, Plimmerton and Pukerua Bay may see waves topping seawalls and flooding the road, he said.
There was a thunderstorm watch and strong wind warning in place for the Wellington region until 8am tomorrow and people could expect strong wind gusts up to 120km/h, some heavy rain, hail and the possibility of more localised tornadoes, he said.
It could lead to flash flooding, damage to trees and powerlines and may make driving hazardous.
"In addition to that we've got a heavy swell warning from midnight tonight and that's going to go through until midnight tomorrow night and so we can expect some rather large waves, about 4.2 metres in height along the west coast running from Ōtaki down as far as Mākara."
People should take a proactive approach and people should avoid coastal areas that were likely to be impacted, he said.
Those with coastal properties needed to shift their cars, or possibly put sandbags in front of areas such as garages to prevent them from being inundated, he said.
The warning also applied to Kāpiti residents in Paekakariki, Raumati South, Raumati Beach, Te Horo Beach and Ōtaki Beach and said to watch out for flooding and debris, and to take care while driving.
Coastal residents should prepare for heavy swell warnings by creating a household plan and having a grab bag ready, in case they needed to leave quickly, Holmes said.
Emergency services are on standby, council staff will continue to monitor the situation overnight, he said.
The Wellington region has had severe weather today with heavy rain, thunder and lightning in the capital and a suspected tornado damaging houses and cars on Kāpiti Coast.