Rain, thunder and wind for the South Island and Wellington - welcome to a summer's day in New Zealand

Heavy rain and thunderstorms are hitting the West Coast in the latest wild weather to hit the country.

A bomb low - a term for a rapidly deepening low pressure centre - is expected to cause severe weather for the next two days.

Surface flooding is affecting several South Island highways and gale gusts of up to 150km/h are expected in exposed Canterbury areas tonight.

Meanwhile, ferry services out of the capital were cancelled this evening and strong winds toppled a traffic light at the corner of Taranaki and Dixon streets in the city centre. A power pole also reportedly blew over on Aotea Quay.

More than 500 houses in Trentham, Upper Hutt lost power around 6pm this evening, which has since been restored.

NZTA is warning drivers in the Wellington and Wairarapa regions to take care as trees and debris could fall on roads.

Wind is expected to peak in the lower North Island around 9am tomorrow morning.

Metservice meteorologist Lisa Murray advised people to tie down trampolines, and said the winds would show up weaknesses in loose roofs.

"Also for the likes of Wellington, where we have some buildings that are being knocked down, the debris could be caught up in winds of this strength," she said.

The Wellington City Council is keeping a close eye on demolition sites across the city.

Trolley buses which were stopped temporarily are up and running again, and trains are operating as normal.

Those flying out of Wellington are being advised to allow extra time to travel to the airport.

Wellington Airport has posted a message on its website, advising travellers to check for any potential delays and leave extra time given the severe weather forecast.

Mark Owen, of NZTA, said winds of this strength can blow vehicles around, especially on State Highway 2 over the Rimutaka Hill.

"Particularly through the cuttings where you get accentuated wind, there's a risk they could get blown off the road or at worse into the path of an oncoming vehicle."

The MetService has also issued a strong wind warning for the Selwyn area, in central Canterbury, forecasting northwest gales gusting to 150km/h in exposed places from late today until mid tomorrow morning.

Further south, the Selwyn District Council fears a repeat of the September 2013 windstorm that caused $75 million damage nationwide, damaging farm equipment, buildings and forestry plantations. Some areas in Selwyn were without power for more than a week.

Council principal rural fire officer Douglas Marshall said people should avoid operating machinery, or anything that could start a fire and to make sure previously lit fires are fully extinguished.

"Strong winds along with hot weather will create an extreme fire risk and fires will be difficult to control due to the extreme winds," he told the star.kiwi website.

He warned that people should avoid driving in strong winds or in the dark afterwards as there could be unseen hazards.

The National Institute of Water and Atmospheric Research said incredible rain totals were expected for the west of the South Island, including in the Southern Alps where almost half a metre of rain was forecast.

Severe weather warnings:

Westland: From 9am today until 3am tomorrow, between 200mm to 300mm of rain is forecast to accumulate in the ranges, with 60mm to 100mm about the coast. These amounts are on top of rain that has already fallen. Peak intensities of 30mm to 40mm of rain every hour from this evening, with thunderstorms possible. Heavy rain expected to ease early tomorrow.

Headwaters of the Canterbury lakes and rivers: From 9am today to 3am tomorrow, about 250mm of rain is forecast to accumulate about the main divide, with 80mm to 120mm spreading 20km further east. Peak intensities of 25mm to 35mm an hour from this evening, with thunderstorms a possibility. Heavy rain due to ease early tomorrow.

Buller: Heavy northwesterly rain forecast from this afternoon until tomorrow morning. From 3pm to 6am tomorrow, 100mm to 150mm of rain is expected to accumulate. Peak intensities of 20mm to 30mm an hour forecast from tonight with thunderstorms possible.

Wairarapa: From late this evening until late tomorrow morning: Northwest gales becoming severe, with gusts of up to 150km/h.

Wellington: From this evening until mid-morning tomorrow: Northwest gales becoming severe, with gusts of up to 160km/h in exposed places.

Marlborough: From late tonight to mid-morning tomorrow: Northwest gales becoming severe gusts of up to 150km/h.

Canterbury north of Ashburton: From late tonight to tomorrow morning: Northwest gales becoming severe gusts of up to 150km/h.

Fiordland, Southland and Stewart Island: Tomorrow morning: severe south to southwest gusts of up to 130km/h in exposed places.

Source: MetService (next severe weather warning due at or before 9pm today)