Hot days: 30C before lunch time, hottest yet to come

It's a sweltering day in the South Island as temperatures in some parts are already hit above 30C before lunch time - and the hottest is yet to come.

Parts of Southland have reached 30 degrees this hour, 32 in Central Otago and as high as 33 in Canterbury, Weatherwatch.co.nz reports.

"The hottest part of the day is still a few more hours away," said Weatherwatch analyst Phil Duncan.

"Incredibly, tomorrow may be even hotter in parts of Southland and Otago as the dry airflow continues."

North Island faces a different type of heat as humidity brings "thicker air", and making lower temperatures feel hotter,

"Humidity levels aren't extreme but it's making temperatures in the mid to late 20s feel more like 30," Duncan said.

Many areas in the upper North Island will have overnight temperatures in the 20s.

MetService has issued a severe thunderstorm watch and is warning that thunderstorms could hit the central and southern North Island on Sunday.

MetService spokesman John Crouch says the combination of another warm humid day and light winds will lead to cloud build-ups, and this could result in scattered heavy showers and thunder.

Areas affected include southern Taupō, Taumarunui, Taihape, inland Whanganui, northern Manawatū, the western ranges of Gisborne andHawke's Bay.

"Some of the thunderstorms may severe with localised downpours of 20-40mm/hr and small hail," Crouch said.

"Rainfall of this intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips."

Motorists are being warned of hazardous driving conditions and poor visibility in the heavy rain.

"The thunderstorm activity should die down by 9pm this evening," Crouch said.

A deep low pressure system moving over New Zealand is also expected to bring rain and wind across most parts of the country this week.

Auckland and the upper North Island can expert a fine and cloudy start, but showers, possibly heavy this afternoon and evening is expected to hit Bay of Plenty, Waikato and Waitomo.

The humid conditions would also see temperatures soar into the high 20s in Invercargill with a forecast of 28C on Monday.

"A ridge over the country retreats slowly eastwards on Monday as a deep low pressure system over the Tasman Sea moves eastwards," MetService says.

"During Tuesday and Wednesday, the low is expected to move east over New Zealand, bringing rain and strong north to northwesterly winds over much of the country. It is followed by a ridge on Thursday."

Rainfall accumulations reaching "warning criteria" are forecasted for Westland on Tuesday through to Wednesday - and likely also for Buller, Nelson and western Marlborough.

MetService said there is also possibility of northwesterly winds becoming severe gale in exposed places of Wairarapa, Hawkes Bay and Gisborne on Wednesday.

"There is low confidence of severe north to northwesterly gales in exposed places of the west coast of the South Island including Nelson and Marlborough and north Canterbury, and in the north and west of the North Island from Northland to Bay of Plenty to Wellington during Tuesday and Wednesday," MetService said.

"But the confidence should increase to moderate for Wellington overnight Tuesday."

Today, it will be mostly fine except for a few showers about the ranges for Horowhenua Kapiti Coast and Wellington.

Mostly fine also for the rest of the North Island but a few showers mainly inland during the afternoon and evening, some heavy with thunderstorms.

Canterbury and Otago started the day with areas of fog and look likely to end with the same this evening.

Weather this week:

Monday: North Island, cloudy periods with a few showers possible during the afternoon. South Island, Mainly fine in the east, a few showers turning to rain south of the Glaciers and Northerly gales in Fiordland.

Tuesday: North Island, cloud increasing with a few showers and possible gale northeasterlies in some areas. Rain developing in Northland late in the day. South Island, heavy falls and cloud increasing in the east, and rain forecasted in the west. Northeasterlies rising to gale is being forecasted for the coasts.

Wednesday: Heavy falls north of Taranaki to East Cape, easing to a few showers. Strong or gale northeasterlies easing. Rain, possibly heavy in the north and west of the South Island, and showers developing in the south.