It's going to be a wet and windy start to the week for many parts of the south and lower North Islands, with thunderstorms and gale force winds expected in parts.

A severe weather warning is in place as the risk of thunderstorms hover over Fiordland and the Westland ranges.

Heavy rain of about 120mm to 180mm, in 12 to 18 hours, is expected to start falling this morning around northern Fiordland and the Westland ranges.

MetService meteorologist Derek Holland said despite the wet weather temperatures remained rather warm for this time of year.

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"The temperature is still quite warm, we don't expect it to go to freezing ... there won't be any snow."

He said those in the areas affected by the worst rain needed to abide by the usual warnings and be aware of the risk from swollen rivers.

Mr Holland said a warning for strong winds that could reach gale force was also in place for parts of Canterbury and the lower North Island.

He said it "might be uncomfortable" flying into Wellington, but he didn't expect it would cause the cancellation of flights.

In general around the country Mr Holland said the weather was unseasonably warm.

"The temperature has remained warmer than expected, the pattern this autumn has been for warmer milder weather."

In Auckland today temperatures are expected to reach a high of 22C, with mostly cloudy periods with a few afternoon and evening showers.

Wellington is expected to reach a high of 18C with cloudy periods and a strong northerly with the potential for gale force winds later today.

Christchurch was forecast to reach an unseasonably warm 23C for the start of May, with gusty northerlies expected around the high country.

NZH