Key Points:

    Gales up to 130km/h expected across Fiordland, Southland and Otago Thunderstorms and rain up to 300mm expected for Westland Possibility of surface flooding and slips

A severe weather warning has been issued for the South Island as heavy rain and strong winds bear down on the country.

MetService has forecast northwest gales up to 130km/h across Fiordland and Southland from late tonight, expected to hit Otago from early tomorrow.

It warned the winds could exceed 130km/h around inland Canterbury, particularly about the high country, tomorrow.

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"Winds of this strength could bring down trees and powerlines, damage unsecured structures and make driving hazardous," Metservice said.

"A period of heavy, thundering rain," up to 200mm, was expected to hit Fiordland early tomorrow.

Thunderstorms and rain up to 300mm was forecast for Westland south of Harihari from early tomorrow, and early Wednesday morning in the ranges between Harihari and Otira.

Metservice warned this amount of rain was likely to cause rivers and streams to rise rapidly, and may lead to surface flooding and slips.