Parts of the Coromandel have received a colossal 170mm of rain overnight, as the start of a low pressure system hits the North Island.

At least two vehicles were trapped in floodwaters last night as torrential rain and thunderstorms lashed the Coromandel Peninsula.

The first thunderstorms hit the holiday hotspot around midnight, bringing 100mm of rain between 11.30pm and 1.30am.

Northland, Auckland, Coromandel and parts of Gisborne, Waikato and the Bay of Plenty are now in the firing line for heavy rain and possible severe gales.

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Dalmeny's Corner, near Whenuakite on the Coromandel Peninsula, where torrential rain caused flooding and closed SH25 overnight. Photo / Georgie Hood
Dalmeny's Corner, near Whenuakite on the Coromandel Peninsula, where torrential rain caused flooding and closed SH25 overnight. Photo / Georgie Hood

As of 9am there had been 171mm of rain in Whitianga, 167mm in Whenuakite and 113mm in Whangamatā, according to Niwa.

People in the upper North Island are being warned to tie down trampolines and take care driving as the area braces for further heavy rain and possible severe gales later today.

Flooding close to Dalmeny Corner in Whenuakite, off SH25, on the Coromandel Peninsula following torrential rain overnight. Supplied photo / Georgie Hood
Flooding close to Dalmeny Corner in Whenuakite, off SH25, on the Coromandel Peninsula following torrential rain overnight. Supplied photo / Georgie Hood

Slips, flooding and fast-rising rivers are possible, forecasters say. Unusual easterly winds could also catch people off guard, and have the potential to damage trees, lift roofs and cut power.

Weatherwatch says there is a 65 per cent chance of isolated power outages today. Eastern Waikato, Great Barrier Island, North Auckland and Northland will be most exposed to the strong easterlies.

MetService meteorologist Tahlia Crabtree said Coromandel's rainfall totals were considered "very heavy" and there was more on the way.

A large high-pressure system expanding east of New Zealand was working with a weak low to both produce rain and stop it from moving away.

The combination of the powerful high and small low meant a sub-tropical noreaster was developing, WeatherWatch head forecaster Philip Duncan said.

Rain clouds would funnel down from the moisture-rich sub-tropics directly into northern and northeastern New Zealand.

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"Rain will line up with certain eastern parts of the upper North Island later, being fed by the blocking high to the east which also stops the rain clouds from moving through - increasing rainfall totals to flood and slip risk criteria for some today," Duncan said.

A severe weather warning was issued for the Coromandel around 10.30pm on Saturday, and another band of thunderstorms passed over early this morning.

Bad weather blocks roads

State Highway 25 between Hikuai and Whitianga - a key route through the Coromandel Peninsula - was closed overnight because of flooding, sending drivers on a detour through the Coromandel township.

The road was reopened around 9am after floodwaters subsided.

A police spokeswoman confirmed there had been "a couple" of reports of trapped vehicles although they were not major incidents.

In Northland, a slip last night also closed State Highway 12 west of Brynderwyn overnight, blocking the route to Dargaville. Motorists were being advised to take State Highway 14 via Maungatapere instead.

The road is now open but motorists are still advised to avoid the area, as a stop-go management system is in place.

Heavy rain warnings increase

MetService has a heavy rain warning in place today for Northland, especially in the east. The forecaster this morning revised the expected rainfall upwards, with 100-130mm of rain now expected between 9am Sunday and 9am Monday. The Kaipara region has already had lightning strikes and rain early this morning.

A heavy rain warning for Coromandel has also been brought forward to 1pm, with increased rainfall totals of 120mm-160mm now expected before 6pm Monday.

Both regions can expect peak rates of 20-25mm/hr, according to MetService.

The Western Bay of Plenty and parts of Gisborne have been added to the rain watch, with heavy rain likely from 1am Monday and throughout the day.

A heavy rain watch is also in place for Auckland from 3pm till 11am Monday, especially in the north and around the Hunua Ranges.

That rain may be welcomed in Auckland in particular, with the city receiving just half its normal rainfall since the start of November. The city's dams are 43 per cent full, compared 77 per cent on average at this time of year.

Strong winds from tonight: 'People may be unprepared'

Both MetService and Niwa are also warning of strong easterly winds that will start tonight and could catch people off guard.

Crabtree said a strong wind watch was in place for all the northern region including Waikato.

"That could be quite significant because winds from this direction haven't been felt for quite a while," she said.

Gale-force easterlies are possible in Northland from 6pm tonight till 6am Monday, while Auckland, Coromandel Peninsula and Waikato north of Huntly will get strong easterlies - potentially severe gales - between 8pm tonight and 2pm Monday.

People with trampolines should tie them down and those driving high-sided vehicles should be aware of the risk.

"A strong wind from the east isn't typical so people may be unprepared," she said.

"The system moving through has been quite changeable in our computer model guidance. As it's been tracking through, we're trying to identify where the most-affected areas will be."

Waikato and the Bay of Plenty could also feel the force of the weather system on Monday.

The winds could be strong enough to cause power outages to exposed areas of the upper North Island, Weatherwatch's Philip Duncan warned.

Eastern Waikato is in the firing line this evening; gusts over 120km/h are possible on the western side of the Kaimai Ranges and the Coromandel Peninsula.

The Hauraki Gulf, Great Barrier Island and north of Auckland could get gusts of up to 80km/h or 90km/h, Duncan said.

"Winds peak overnight tonight/Monday morning and ease into Monday as they slide down the North Island in a similar fashion and peak winds start to drop," he said.

Today's weather

Whangārei A few showers turning to rain in the morning, heavy at times. Easterlies becoming strong. High 17C, Low 15C

Auckland Rain setting in this morning, heavier from afternoon. Easterlies becoming strong. High 17C, Low 14C

Hamilton Cloud increasing. Rain developing in the evening. Easterlies becoming strong and gusty from evening. High 16C, Low 13C

Tauranga Becoming cloudy. Rain at times from evening. Easterlies strengthening. High 17C, Low 13C

New Plymouth Cloud increasing. Light rain developing at night. Easterly breezes. High 17C, Low 11C

Napier Mostly cloudy with a few showers. Easterly breezes. High 16C, Low 12C

Whanganui Mainly fine with increasing high cloud. Easterly breezes. High 19C, Low 11C

Wellington Mainly fine. High cloud increasing in the evening. Southeast breezes. High 15C, Low 10C

Nelson Fine with evening high cloud. Light winds. High 15C, Low 8C

Christchurch A fine start, with frost in sheltered places. Cloudy periods developing morning. Northerlies. High 13C, Low 7C

Dunedin Mainly fine. Cloudy about the coast at night. Northerlies developing in the morning. High 14C, Low 6C