MetService has issued a severe weather watch for the West Coast and Southern Alps.
Heavy rain is expected to move across the Tasman Sea and reach southern New Zealand late this afternoon.
An extended period of heavy rain has been forecast for Fiordland, Westland, Otago, and Canterbury.
Strong to gale northerlies, and heavy rain are forecast to move slowly northwards over the South Island from late this afternoon through to Sunday.
Heavy rain is expected to set in over Fiordland later this afternoon, reaching Westland either tonight or tomorrow morning.
The heaviest falls are expected for Westland south of Otira and for northern Fiordland, especially in the ranges, beginning this afternoon.
A significant amount of spill-over rain is also likely for the headwaters of the Otago and Canterbury Lakes and Rivers, although this is most likely tomorrow occur tomorrow and over the weekend.
Given the prolonged nature of the rainfall, total accumulations may exceed 500mm in the ranges of Westland.
At this stage, MetService is calling the weather watch a "significant weather event," for the South Island's West Coast.
Motorists have been advised that the combination of strong winds and heavy rain are likely to make conditions hazardous.
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