An "atmospheric river" has descended from the tropics bringing warm, humid air to the west and scorching temperatures out east.

For Aucklanders and many on the North Island's west coast it has meant thick, tropical air with high humidity - which means a terrible night's sleep: overnight the temperature in Auckland only dropped to 17C.

Meanwhile those on the South Island's West Coast were in for an extremely wet morning.

MetService meteorologist Nick Zachar said so far the highest rainfall levels were about Franz Josef and Hokitika where 50mm had fallen overnight. Some spots could receive another 300mm of rain through the day.

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The only part of the country to escape any of the wet will be the North Island's east coast, from Gisborne south to the Wairarapa, parts of which could crack 30C today.

The hot and dry conditions there were driven by the foehn effect.

It occurs where a moist air mass moves across a landmass, in this case the North Island's "spine" of mountain ranges, dumping moisture but also producing heat, leaving behind warm and dry air.

Zachar said conversely, western areas were in for a wet day.

"A whole bunch of moisture, what we call an atmospheric river, has come down from the tropics. It is bringing a lot of sea fog, drizzle and low cloud to the North Island's west coast, and high humidity.

"For the South Island's West Coast and Nelson, it is bringing some very heavy rain."

The active front was forecast to move slowly across the South Island today.

The heaviest rain was expected in the western and southern districts of the South Island and also about the headwaters of the Canterbury and Otago lakes and rivers.

Heavy rain warnings were in place for many of those areas through to tonight, with some spots expecting to receive up to 300mm of rain. There was potential for flooding and slips.

The NZ Transport Agency has issued several road warnings in the South Island.

In Nelson, State Highway 60 from Riwaka to Takaka has a warning in place for heavy rain, and in Canterbury parts of SH73 and SH7 have warnings of strong winds, meaning care was required by all high-sided vehicles and motorcycles.

On the West Coast there was a risk of flooding along SH6 from Hokitika to Haast.

Strong winds were also starting to ramp up about the Canterbury high country, Wellington and southern Wairarapa. Gusts could reach 150km/h in exposed parts of Canterbury, and 130km/h in other affected areas, and were forecast to peak around midday.

Ferries in Wellington harbour are running with restrictions this morning due to worsening weather.

Metlink said ferries would be departing Days Bay at 7.45am and 8.45am.

Severe northerly gales are forecast for the capital today with gusts of 130km/h in exposed places.

The rain would spread slowly east over the South Island today, with a cooler southerly change behind it. Canterbury will see a drastic change in temperature, dropping from a high of 27C today to just 12C tomorrow, with heavy rain.

On Friday the slow moving front and low over central New Zealand would continue to deliver rain, with further heavy rain forecast for Buller and western Nelson, and moderate confidence about the remainder of Nelson, Marlborough, northern parts of Canterbury, Wellington and Kāpiti/Horowhenua.

Rain would spread across most of the North Island through the day, with less expected in eastern areas.

Heavy snow was also expected about northern parts of the Canterbury high country above about 800 metres, with lesser amounts down to about 500m, affecting higher roads and passes in this region.

The rain would gradually ease to showers during Friday and Saturday as the low and frontal system moved away to the east.

Late Saturday a large ridge of high pressure would spread over New Zealand and remain slow moving through to Monday.

However, several weak fronts would affect the far south over this period, though nothing too severe was expected.

Today's weather

Whangārei

Cloudy periods, drizzle at times until evening. Northwesterlies. 23C high, 15C overnight.

Auckland

​ Cloudy periods, drizzle at times until evening. Northwesterlies. 22C high, 15C overnight.

Hamilton

Mostly cloudy, drizzle at times until evening. Northwesterlies. 23C high, 14C overnight.

Tauranga

Often cloudy, drizzle at times. Northwesterlies. 21C high, 15C overnight.

New Plymouth Cloudy, drizzle at times. Northerlies developing morning. 18C high, 14C overnight.

Napier Sunny. Warm, gusty northwesterlies. 29C high, 18C overnight.

Whanganui​ Cloudy periods. Northwesterlies. 24C high, 16C overnight.

Wellington Cloudy, with patchy drizzle, mainly north of the City. Strong northerlies, rising to severe gale in exposed places and gusting 130km​/​h. 17C high, 14C overnight.

Nelson Occasional rain, persistent from afternoon. Northerlies. 19C high, 16C overnight.

Christchurch Rain from midday with strong northwest changing cool southerly. 27C high, 8C overnight.

Dunedin Rain, chance heavy, easing evening. Southerlies. 15C high, 8C overnight.