Western regions of the South Island could be in for some extremely stormy weather later today with squally thunderstorms and a chance of tornadoes.

MetService has issued a heavy rain warning for parts of Fiordland, Westland and Southland and says there is a moderate risk of thunderstorms over parts of Westland and Buller later tonight.

Duty forecaster Claire Flynn said up to 70mm of rain was expected to fall in the far south over a short period of time.

The skies over Westland and parts of Buller were expected to light up with thundery activity late tonight and into tomorrow morning.

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An alert had been issued for a moderate risk of thunderstorms and a chance of small tornadoes.

The North Island can expect today to be fine today and parts of tomorrow.

MetService duty forecaster Jasmine Kennedy said it would be fine in northern and eastern parts of the North Island with temperatures sitting in the early 20s.

Auckland was expected to reach 21C, Tauranga 22C and Napier 23C.

The lower North Island could expect to cloud over with drizzle starting to fall later today.

MetService also said the squally, thundery showers could be accompanied by strong winds measuring between 90 and 110km/h.

The wet weather warnings would be revised later today.