Severe gale-force winds are on the way for much of the country, while the west of the South Island is in for a drenching.
MetService is warning rivers could rise rapidly and trees and powerlines could be brought down.
The wind and rain precede an active front expected to approach the South Island tomorrow, then move slowly northeast to reach the lower North Island late on Wednesday, and the upper North Island late on Thursday.
The heaviest rainfall is expected to be in the Westland ranges where up to 250 millimetres could fall in 24 hours from 8am tomorrow.
The strongest wind speeds predicted are gusts of 140kmh in inland Canterbury and the Canterbury Plains north of the Rangitata River tomorrow afternoon and evening.
Christchurch can expect a high of 20 degrees Celsius tomorrow, followed by 18C on Wednesday. Winds will ease on Wednesday.
Northwesterlies gusting up to 120kmh are forecast for Fiordland, Southland, Otago and Canterbury from late tomorrow morning. They will ease tomorrow evening in the more southerly regions, and in south Canterbury from early Wednesday and north Canterbury by Wednesday afternoon.
In eastern and northern Marlborough, Wellington and Wairarapa winds gusting to 120kmh are forecast from late tomorrow afternoon. They are expected to ease in Marlborough on Wednesday evening but not until early Thursday in Wellington and Wairarapa.
"Winds of this strength are likely to bring down trees, powerlines and make driving dangerous for motorcycles and high-sided vehicles," MetService said.
Heavy rain and thunderstorms are forecast for the West Coast from Fiordland to Buller, spreading east over the Otago and Canterbury headwaters tomorrow and Wednesday.
Along with the forecast for up to 250mm to fall in the Westland ranges, 200mm could fall just east of the main divide, and up to 150mm within 30km of the divide.
Up to 200mm could fall in the ranges of Buller and the Tasman Mountains in the 24 hours from 10pm tomorrow, with up to 150mm about the Nelson Lakes National Park, MetService said.
"This is a significant amount of rain and people in the affected areas are advised that rivers are likely to rise rapidly with the risk of flash flooding."
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