The hot weather seems endless, with Kiwis around the country flocking to the beach on the weekend. Even karate classes were held at the water's edge as Santa and his students attempted to cool off.

But there's no guarantee this hot spell will continue, forecasters say.

Instead, this week is set for a "seesaw" of hot and cooler weather as fronts nibble away at the high pressure ridge that's sitting over the country.

A front is expected to cool some cities down today by as much as 10C, while thunderstorms could pelt parts of the North Island, bringing hail and dangerous driving conditions.

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A front from the South Island is set to reach the north today, briefly bringing rain, according to the MetService.

But the system will move fast - reaching Northland by the evening - and rain is unlikely to bring any drought relief, except in localised areas affected by thunderstorms.

Thunderstorms are possible this morning in Nelson, Buller, Taranaki and Whanganui, while Kapiti, Horowhenua, Manawatu, Taihape, Taupo, Taumaranui and Waitomo could hear thunder before lunchtime.

In the afternoon western areas from Taumaranui to Auckland are at moderate risk of thunderstorms, while the early evening could see thunder from Taupo across to Gisborne and Hawkes Bay.

There is a low risk some storms could become severe, the MetService said.

Later this week more fronts will bring rainfall to the southwest and cause temperatures to "yoyo", they said.

Christchurch was forecast to bounce from 19C today to 26C tomorrow, 19C on Wednesday and 29C on Thursday.

"As fronts make inroads onto New Zealand we can expect more variable weather conditions", meteorologist Tom Adams said.

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"The biggest fluctuations between cool and hot, wet and dry, will be in the south. Further north the fronts will have weakened, but we still expect the coming weather to be more changeable than during the last few weeks.

"With the holidays approaching and people starting to make outdoor plans, it is worth remembering that the weather does change, and it always pays to keep an eye on the latest forecasts. Don't take fine weather for granted."

MONDAY'S FORECAST

WHANGAREI

Showers, afternoon southwesterlies. High 26C, Overnight 17C

AUCKLAND

Afternoon showers, chance of thunderstorm. High 25C, Overnight 17C

TAURANGA

Cloudy, showers from afternoon. High 26C, Overnight 15C

HAMILTON

Rain, possible thunderstorms in afternoon, clearing. High 25C, Overnight 13C

NAPIER

Rain from midday, easing evening. High 26C, Overnight 12C

WELLINGTON

Morning rain then fine. Northerlies turn strong southerly. High 19C, Overnight 11C

CHRISTCHURCH

Early rain clearing. High 19C, Overnight 9C

DUNEDIN

Fine. High 16C, Overnight 11C

QUEENSTOWN

Fine, chance of showers in afternoon. High 21C, Overnight 10C