Sunny conditions are forecast over the top half of the country over the next few days.

But showers and colder temperatures are expected over the South Island heading into the weekend.

Aucklanders will wake up to a mostly sunny day today with a temperature high of 21C and an overnight low of 13C. Conditions will be mainly fine tomorrow, with light winds.

Hamilton is in for a sunny day as well, with afternoon sea breezes and a high of 21C. It will be cloudier tomorrow, however, and the temperature drops a bit to a 19C high.

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Tauranga and Napier both look mainly fine today and tomorrow, with temperature highs in the early 20s. Napier will be particularly warmer tomorrow, with a temperature high of 26C expected.

Those in the Capital will be happy to know it will be sunny in Wellington today, with a 16C temperature high. The day will be cloudy tomorrow, however.

New Plymouth will also get sunshine today, but there are cloudy periods forecast tomorrow and a chance of shower, as well as a 17C high expected.

A bit of a cold snap has hit some areas today.

WeatherWatch head analyst Philip Duncan said a southerly airflow across the country was the cause for some of the lowest temperatures as of 6.30am today.

Hamilton recorded 6C, Taupo 4C, Rotorua 6C and Hastings 5C. Upper Hutt recorded 4C and Twizel 0C.

MetService said a cold front was due to move northwards over the South Island tomorrow and would weaken at it moved over the North Island on Saturday.

That would mean for cooler temperatures.

Nelson gets sunny conditions today and tomorrow, with temperatures between 19C and 22C. Blenheim has similar conditions.

Christchurch is mostly sunny and has a high of 17C high today and an overnight low of 5C.

Further south, Dunedin will get cloudy periods and a few showers today. Showers, at times heavy, are forecast tomorrow; and there is also hail.

There will be a chance of showers this morning in Queenstown and showers are expected tomorrow, with a high of 19C.

While Invercargill will get chilly temperatures - 15C today, 14C tomorrow and 13C on Saturday - and showers, possibly heavily. Strong westerly winds are also forecast over the next few days.

"The front is expected to bring a brief period of heavy rain to the west of the South Island on Friday, but warning amounts of rain are not expected at this stage.

"Temperatures are forecast to be unseasonably cool for many places in this southwest air-stream.''

Another cold front is also forecast for Monday over the South Island - again bringing with it a period of heavy rain into the west.

MetService said there was "low confidence'' that there would be warning amounts of rainfall for Fiordland and southern Westland.

The front was expected to weaken as it moved north and unlike tomorrow's front was not forecast to bring cold air with it.

Holiday makers heading for the beach today are being warned about big swells down the East Coast.

Swells of up to 5m were predicted on the East Coast of both islands.

The waves are set to peak around Banks Peninsula this morning, in Wellington and Wairarapa around midday, and Gisborne and the Mahia Peninsula late afternoon.

The swells are being generated by a deep low to the south east of the country.

Meteorologist Lisa Murray said in close to that low there are storm force winds of 50 knots, which is about 93km/hr mean speed.

She said we do get big swells in New Zealand so it was not particularly extraordinary.

However, she said given the time of year, many needed to remember there were lots of people down the coast and out in the sea in small boats because of the holidays.

Murray said the swell has a decent period and a period gives you an idea of the strength of the swell.

She said this is a 13 to 14 second period, which meant it has a good push to it as the swell comes through.

Those heading out on the water needed to be aware and to also take precautions, she said.

Parents also needed to be aware that the swells were quite large and strong; and to therefore make sure small children did not play close to the shoreline.

There was a possibility of damage to southern facing coastlines in Wellington, she said, and there will be some swell through Cook Strait which could make ferry crossings a little bumpy.

But some who will be welcoming the swell would be surfers.

Today's forecast:

• Kaitaia: 22C high. Sunny.
• Auckland: 21C. Mostly sunny. Light southwest dying out.
• Hamilton: 21C. Sunny with afternoon sea breezes.
• Tauranga: 22C. Sunny with light winds and afternoon sea breezes.
• Napier: 23C. Fine. Southerlies dying out in the evening.
• Wellington: 16C. Sunny. Southerlies dying out in evening.
• New Plymouth: 19C. Sunny. Westerlies developing in morning.
• Nelson: 19C. Sunny with southwest breezes turning northeasterly.
• Blenheim: 19C. Sunny. Westerly breezes turning northeasterly.
• Christchurch: 17C. Mostly sunny. Strong southwest eases.
• Dunedin: 16C. Cloudy periods and a few morning showers.
• Queenstown: 20C. Chance early shower, otherwise sunny.
• Invercargill: 15C. Cloudy periods, a few morning showers.