The big downpour is here. Two weather systems are spreading across New Zealand - and much of the country is bracing for a days-long deluge, prompting flood warnings in many places.

An active front was moving slowly across New Zealand from Wednesday, and the MetService said it would continue in the north and west into Friday.

The heaviest rain was forecast for the ranges of Westland, south of Otira, and Nelson from about Motueka westwards - with 150 to 250mm of rain expected to fall into early Thursday.

NINA HINDMARSH/STUFF The Washborne Scenic Reserve sign at Onekaka in Golden Bay was nearly underwater.

MetService meteorologist Tom Bell said a solid band of rain would be "setting up camp over the North Island", and more rainfall was expected than the major storm that battered the country on January 5.

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Northland, Auckland and north Taranaki were bracing for 100 to 150mm of rain to fall from late Wednesday until early Friday - and about 250mm was set to fall at Mt Taranaki.

NINA HINDMARSH/STUFF Flooding in paddocks in Ferntown in Golden Bay, from the Aorere River.

The MetService warned heavy rain could cause streams and rivers to rise rapidly. "Surface flooding and slips are also possible and driving conditions may be hazardous."

A high to the east of the country was set to block the front bringing the rain, keeping it in place over much of the nation.

On Wednesday afternoon, the rain was soaking the Nelson region, and it later spread to Wellington.

NINA HINDMARSH/STUFF 207mm of rain has fallen at Anatoki; making One Spec Rd impassable.

NELSON DRENCHED

A MetService meteorologist said from midday to 3pm, rain was falling at about 20mm per hour near Takaka, in the Golden Bay.

Nelson Tasman Civil Defence has sent a team to the area and is warning of possible river bank breaches. They are telling people not to freedom camp in the area.

Duty emergency manager for Nelson Tasman Civil Defence Laurie Adams said Golden Bay was currently the main area of concern for response crews, who would be liaising with police, fire and Department of Conservation staff around the Nelson region as the situation developed.



"[The weather] is similar to what we had in the weekend but a lot more intense over a shorter period of time so the impacts are probably going to be a bit larger," Adams said.

The heavy rain forecast overnight would coincide with high tides at around midnight.

Adams said the NZRT-2 response team had arrived in Golden Bay around 3pm and was likely to remain there until midnight, but may stay overnight if necessary.

NIWA An image from Niwa shows a siphoning of moist air down from the tropics.

Surface flooding has been reported on roads near Rockville, Collingwood, Anatoki and at Onekaka.

According to the Tasman District Council website, Anatoki in Golden Bay has seen 270mm of rain today.

Due to flooding near the Little Onahau River Bridge, near Puramahoi, New Zealand Transport Agency has put a temporary 50kmh speed restriction in place for motorists travelling in both directions along State Highway 60 between Takaka and Collingwood.



No alternative route is in place, however road users are advised to take extra care.

MetService

MetService Meteorologist Amy Rossiter said the rain would persist for quite a long time.

"It will probably stay like this today and won't ease up until tomorrow morning," she said.

"We are expecting the peak rain rate to be this afternoon and this evening. We have got some strong northerly winds at the moment which will strengthen ahead of the front."

BRADEN FASTIER / STUFF A band of rough weather hits the Nelson region which is to be followed by a deluge in the afternoon.

WELLINGTON'S WEATHER TURNS

Rossiter said the front was going to move right across the whole country.

"Most of the North Island from Northland to Wellington is going to get heavy rain."

METSERVICE Rain hits the West Coast, Tasman and Taranaki regions in this MetService radar image from 9.21am Wednesday.

By 3.30pm, the rain had begun to hit Wellington. MetService took to Twitter to warn commuters: "If you're heading home, don't hang around".

AUCKLAND BRACES FOR 24 HOURS OF RAIN

Aucklanders are expecting a very wet Thursday, with heavy rain from sunrise.

Auckland Council's public information management spokeswoman Jo Glasswell said while Civil Defence would be "keeping an eye on weather", no precautionary measures had been put in place.

"We do encourage the public to clear their gutters and drains before a lot of rain, and make sure they drive to the conditions," she said.

Auckland Transport spokesman Mark Hannan said the heavy rain forecast was unlikely affect public transport on Thursday. He said ferries and bus routes only tended to be thwarted by king tides and gale force wind.

METSERVICE Heavy rain covers parts of both the North and South islands in this MetService rain forecast map for 7pm Wednesday.

The Gulf Harbour Ferry had been replaced by a bus service on Wednesday due to strong wind.

Hannan said Auckland Transport would make the call on Thursday morning on whether it would be operating.

SEVERE WEATHER WARNING

MetService says rain will be heaviest in the ranges of Westland, and in Nelson and Motueka - with 150 to 250 millimetres of rain expected to fall from midday on Wednesday. Rain was falling in parts of the West Coast shortly after 11am.

Downpours in the west of the North Island could see up to 250mm fall around Mt Taranaki, and up to 120mm north of Eltham.

"A large area of high pressure to the east & low pressure to the west are like two cogs working together to siphon moisture southward from New Caledonia" weather agency Niwa said in a tweet.

The forecast adverse conditions closed Auckland's Gulf Harbour Ferry service on Wednesday, and the service was been replaced by buses. The Rangitoto Island Ferry is also affected and will only operate in and out of Islington Bay.

The heavy rain in Auckland is expected to intensify on Thursday, where isolated falls could bring up to 120mm of rain.

In Northland, especially north and west of Kaikohe, up to 150mm could fall on Wednesday and into Thursday.

The heavy rain - coming from an active front moving slowly across New Zealand - could cause streams and rivers to rise rapidly, and bring surface flooding or slips.

The rest of the country - particularly Taupō northwards and the Bay of Plenty - is also likely see rain throughout the day, giving some a much needed break from the long sunny days.

However, in Southland, a record-breaking heat wave has left a trail of dead trout across Southland.

High temperatures combined with low water levels have been causing fish to die.

WHAT'S COMING NEXT?

A MetService meteorologist said rain would ease through Friday - but it wouldn't be over.

"There's going to be a few showers on Saturday [and] more for the North Island on Sunday.

"The weather system we are getting at the moment is ... fairly standard. What's making it continue for a few days is there's a high that's [east of] the country.

"The front is being blocked and not allowed to go anywhere."