Heavy rain is forecast to hit much of New Zealand ahead of a mainly fine Christmas Day on Saturday.

MetService has issued a severe weather warning for Taranaki, Waitomo, Taihape, Taumarunui, Wairarapa, Wellington, Marlborough, Nelson, Buller, Westland, Fiordland, Canterbury, Otago.

Forecaster Oliver Druce said a cold front should sweep across the South Island this morning and onto southern North Island areas later today.

The front is preceded by a strong very moist northerly flow and followed by strong northwesterlies.

Heavy rain was this morning falling in many northern and western parts of the South Island, although MetService said this should ease or clear this afternoon. Another front should bring further heavy rain to the west of the South Island tomorrow morning.

This afternoon and evening heavy rain is expected to spread to parts of Taranaki, Waitomo, Taumarunui and Taihape. MetService said the front is expected to stall over these areas tonight and tomorrow morning, with the rain continuing through much of tomorrow.

Northwest gales are also expected to become severe in Wellington this afternoon, before easing this evening.

Slip

State Highway 94 from Te Anau to Milford Sound is closed following a slip, while NZTA is warning drivers to be cautious on SH35 Maraenui Bluff detour, where there has been a slip, and on SH6 from Harihari to Haast due to flooding.

People are also advised not to swim in rivers following heavy rain due to contamination from storm water runoff, with the Nelson District Council advising not to swim for at least 36 hours after rainfall that leads to any discolouration of the water.

The recent rainfall has also seen water restrictions in Kaitaia, Opononi and Rawene temporarily lifted by the Far North District Council.

Hot and humid

While it may again be wet today for much of the country, it will also continue to be warm.

WeatherWatch.co.nz said hot, humid, air will remain over the North Island today, with sunny spells adding to the heat, while strong, hot and dry northwesterlies move into eastern and southern parts of the South Island.

Auckland, Whakatane, Hamilton, Tauranga, Wanganui, Gisborne, Napier, Hastings, Masterton and Palmerston North are all forecast to get highs of 27 to 29 degrees Celsius today but coupled with humidity some areas may "feel like" the mid 30s, WeatherWatch said.

Meanwhile, highs into the late 20s are expected right down the east coast of the South Island and even about Southland, while inland areas may climb into the mid 30s, however humidity will be fairly low.

WeatherWatch predicts the high humidity and heat experienced in the past week in the North Island should drop significantly by Christmas Eve.

Christmas Day

On the big day, WeatherWatch has a welcome forecast for most of the country.

In Northland, Auckland, the western Coromandel Peninsula, Waikato, Waitomo, Central Plateau, King Country, Taranaki, Whanganui, Horowhenua and Kapiti, WeatherWatch forecasts sunny and cloudy periods, with light southwesterlies north of Taranaki and more westerly and brisk winds south of Taranaki. Highs of 19 to 24 are expected.

For the eastern Coromandel Peninsula, Bay of Plenty and Rotorua it is forecast to be sunny with light southwesterlies and highs between 23 and 26.

In Gisborne, Hawkes Bay, Wairarapa and Nelson it is expected to be sunny with light winds, with highs around 23 or 24.

It is expected to be mostly sunny with gusty northwesterlies easing for Wellington and Marlborough, with highs of 20 on the coast and 24 inland.

The West Coast has a 20 per cent chance of showers, especially in the south, otherwise it is forecast to be sunny with cloudy periods and southwesterlies brisk at times but easing. A high of 18 is expected.

In Canterbury and Central Otago it is forecast to be mostly sunny, with westerlies easing. Highs of 22 on the coast and 27 inland are expected.

For coastal Otago cloudy periods and strong westerlies in exposed areas are expected. There is a 30 per cent chance of morning showers in South Otago. Highs of 22 to 24 are expected.

There is a 60 per cent chance of showers for Southland, with conditions cloudy with strong westerlies. A high of 17 is forecast.