About half-a-dozen homes flooded in the South Island overnight.

Fire and Emergency (FENZ) crews responded to flooded homes in Greymouth, Cobden, Brunner and Runanga in the Grey District.

Southern shift manager Lyn Crosson said they also responded to a slip on the road in Cobden.

Crosson said they were gearing up for more heavy rain and possible flooding this afternoon.

Cobden resident Jonathan Stark says his rain gauge shows more than 120 millimetres of rain fell in about three hours overnight.

"It was pretty wild weather. It started about three o'clock with heavy rain and then thunderstorms as well. We had 45 minutes of a proper downpour," Stark said.

"Our weather station, we checked this morning and it said we had 129 mills of rain in under three hours.

"We thought the water would have disappeared overnight like it normally does, but we woke up this morning to get ready to go to work and found what we found."

Water was still halfway up the tyres of his ute.

His garage was also still under water and he expected his spa pool would also be ruined.

Stark expected the damage to his property could cost a couple of thousand dollars.

He had never seen anything like Friday morning's downpour in his five years on the West Coast.

"I've never seen nothing major like this," Stark said.

"I've seen rain - it rains on the West Coast, but nothing so quick and furious."

His cat remained trapped under the house as they could not access it with the water still surrounding the property, but he was pleased his cat had survived the downpour.

"I can hear the cat crying out to get help but we just can't get to it," Stark said.

Surrounding streets were under water.

"It just happened so quickly," he said.

Char Stevens said she was still trapped in her Cobden home.

"We can't go anywhere - that's for sure. We're stuck," she said.

"It's come down from the bonnets of the cars to the tyres now. We're raised so the house is all right but when I waded out before to get wood it was basically up to my knees."

Stevens was woken by thunder early this morning, but it was not until she got out of bed later on that she realised her street looked like a river and water was up to the bonnets of cars.

"All I heard at 3am was the massive thunderstorm," she said.

"I went back to sleep and then my flatmate, when I got up, was like 'come outside'. We opened the door and we couldn't go any further because the water was right up there.

"When it got a bit lighter we could see that it was fully flooded. It's still flooded now but it looks like it's slowly receding. It's just crazy."

More to come

A heavy rain watch had been in place for the Westland Ranges from 8pm on Thursday until 3am on Friday, with heavy rain forecast to move northwards over the North Island on Friday.

West Coast Emergency Management said on social media it was a "cloud burst style rain event".

"Please take care as first light arrives."

Images uploaded by Grey District Council showed surface flooding in Greymouth.

State Highway 7 was closed between Greymouth and Stillwater. The New Zealand Transport Agency advised road users to avoid the area or delay their journey if possible.

The council said parts of Wickes Street, Richmond Street, Bright Street, Barkley Place, Peel Street, and Sturge Street, all had high surface water, and there were reports of a tree down on Taylorville Road.

Heather Keats with details of this weekend’s cold snap

MetService head of weather Heather Keats told Morning Report the heavy rain would head up the west of the country over the next couple of days.

"We've got a bunch of fast-moving features and fronts tied up and that will be thunderstorms, snow, heavy rain, maybe even some small tornadoes and water spouts. And that'll race up and over the country - the North Island - tomorrow, and then it kind of races away.

"But then we're left with a very cold southerly flow. So we've got every type of weather coming over the next sort of 48 hours."

Localised clean-up

Grey District Mayor Tania Gibson said the rain had eased for the moment, but more showers were expected later in the day.

"Our crews have been out since two o'clock this morning, and now that we're in the light of day, there's a lot of assessment going on, there's a lot of debris to clear, there's still road closures in the area and we're trying to find out who's been affected the most.

"But... some of those outer-lying suburbs have been affected and we had a lot of rain come through on the catchment on the hills, which has brought a lot of rain down into town and to those areas and which has brought a lot of debris down as well."

She said there would be a reasonably significant clean-up ahead.

"I was at home, but I was up because we had some poor little lambs in the paddock outside that were crying their hearts out. So, but we do have rain events quite often, but it's hard to assess until you start getting the reports because sometimes, nothing happens, but when they happen in those isolated areas, it's very quick.

"But where I was there wasn't a lot, and driving into town this morning there wasn't a lot."