A deluge of heavy rain has flooded part of an Auckland motorway as thunderstorms begin to roll in over the North Island.

Waka Kotahi NZ Transport Agency said the right lane along State Highway 1 near the Mt Wellington on-ramp is blocked due to flooding.

Motorists are being advised to expect delays.

MetService has issued a severe thunderstorm warning for Auckland, Kaipara, the Far North and Whangārei.

Thunderstorms have been recorded or are expected in Auckland CBD, Howick, Manukau, South Auckland, Auckland Airport, Helensville, Kaukapakapa, Parakai and Makarau.

In the north, thunderstorms have been detected in Dargaville, Purua, Moengawahine, Maungatāpere, Tangiteroria, Te Kopuru, Okahu, Titoki, Maungakaramea and Tokatoka.

“These thunderstorms are expected to be accompanied by torrential rain.”

 “These thunderstorms are expected to be accompanied by very heavy rain and large hail.”

MetService has also issued a severe thunderstorm watch for Northland, Auckland, Waikato, Waitomo, Taumarunui, Bay of Plenty, Rotorua, Taupo between 2pm and 9pm.

“These thunderstorms will bring localised heavy rain and hail.

“Large hail can cause significant damage to crops, orchards, vines, glasshouses and vehicles, and make driving conditions hazardous.”

Dark grey clouds loom amid the sunshine on Boxing Day in Manurewa, Auckland. Photo / Supplied

MetService is warning that some downpours could be severe and cause surface or flash flooding.

“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.

“Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain.”

There is also a risk of small and very localised tornadoes forming.

Meanwhile, much of the country is in for a summer scorcher this week with temperatures on the rise.

MetService said a ridge of high pressure is driving temperatures up and Wanaka could reach 30C tomorrow.

Elsewhere, Hamilton and Whanganui are expected to be the hottest areas today with a forecast of 26C.

Tomorrow, Hamilton’s forecast high is 27C while Auckland’s is 26C.

“Remember in the sun it will feel even hotter than our forecast highs, so keep sun smart and hydrated while you’re enjoying all that,” MetService said.