Weather: Large hail, thunderstorms forecast across New Zealand

People should prepare for stormy, wet weather, as they head out to Boxing Day sales.

Several weather watches have been issued as thunderstorms are expected for the upper North Island and southeast of the South Island.

MetService has issued a severe thunderstorm watch for Christchurch, Canterbury Plains, Canterbury High Country, North Otago, Central Otago, Dunedin, Clutha and Southland from 1pm until 8pm on Friday.

It said there was a possibility of hail larger than 20mm in these areas.

"A cold unstable air-mass affects parts of the South Island today. For coastal Canterbury from the Banks Peninsula southwards, eastern Otago and Southland there is a moderate risk of thunderstorms. These storms may become severe this afternoon and evening with hail, larger than 20mm."

Meanwhile, up north, a complex trough embedded in a moist unstable air-mass was expected to move east across the upper North Island, the forecaster said.

It issued a severe thunderstorm watch for Northland, Auckland, Great Barrier Island, Coromandel Peninsula, Bay of Plenty and Rotorua from 2pm until 8pm on Friday. Gisborne is also under a watch from 7pm until 11pm Friday.

MetService said the downpours could bring hourly rainfall amounts of 25 to 40mm an hour.

Heavy rain watches have been issued for parts of Bay of Plenty, Taranaki Maunga, Tararua Range and Wellington.

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