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A western front followed by a low-pressure system meant everywhere would get some wind and rain this week, MetService Meteorologist Andrew James said.
The heaviest falls would probably be on the west coast of the South Island, where heavy rain watches and warnings were in place.
Central and southern parts of the North Island were also in line for heavy rain, James said.
Strong northwest winds were also expected especially near the Southern Alps today.
The NZ Transport Agency has issued a strong wind watch for SH8, SH8A, SH6 and SH6a in the Central Otago, saying extra care was especially required for high-sided vehicles.
The combination of heavy rain and strong winds at high tide, along with low air pressure could cause some coastal flooding tonight for low-lying areas of Nelson, Buller, and Westland, according to MetService.
Heavy rain in Westland and Fiordland could also see streams and rivers rising rapidly, with surface flooding and slips possible.
Nelson, the Buller and Westland ranges, the Otago headwaters and the Canterbury headwaters north of Arthur's pass were also under heavy rain watches throughout today and early morning Wednesday, according to MetService.
The MetService has also issued road snowfall warnings for Crown Range Rd and Milford Rd where small amounts are expected to settle and Lindis Pass where "little if any snow is expected to settle on the road."
As of 8.30am the Milford Rd was closed between East Gate (Hollyford) and West Gate (Chasm) due to the risk of avalanches.
Strong winds in Canterbury's high country and Fiordland could damage trees and powerlines, the forecaster warned.
Southern Lakes, Central Otago west of Alexandra, and Southland also had strong wind watches in place.
Weatherwatch said the low would get deeper over Wednesday and Thursday, making for unsettled weather and bringing the possibility of thunderstorms, squalls, heavy rain and snow.
By Friday that low would fall apart and by Saturday conditions would begin to improve for most of New Zealand.