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The MetService has issued orange weather warnings for parts of Otago, Southland and Fiordland this weekend.
The warnings mean heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible, and driving conditions may be hazardous.
A MetService spokesman said a large high was drifting eastwards yesterday, allowing an active front to move on to the South Island today.
"This front is preceded by a strong moist northerly flow, bringing heavy rain to Fiordland, Westland, Buller and Nelson.
"The biggest rainfall accumulations are expected about the Westland ranges, where up to 150mm of rain could fall."
He said Fiordland could expect 70mm-90mm of rain between 6am and 6pm today and peak rates of 15mm-25mm per hour this afternoon.
In addition to the heavy rain, north to northwest gales are forecast to affect Fiordland, parts of Southland and Otago, and the Canterbury high country.
He said the gales could be severe at times, and wind gusts could reach 120kmh, particularly in the Canterbury high country between 9am today and 1am tomorrow.
Strong wind gusts could damage trees, power lines and unsecured structures.
It may also make driving hazardous, especially for high-sided vehicles and motorcycles.
Tomorrow, a trough was expected to move on to Fiordland and Westland, bringing further rain, and the orange warning was likely to be extended, he said.