Heavy falls are forecast in the ranges from late Friday into Saturday for the regions.
"We could see over 300mm in the ranges of Westland later this week, and possibly 50 to 70mm in the ranges near Nelson and northern Marlborough. This amount of rain wouldn't normally cause too many problems in Nelson and Marlborough but because they have already had extreme amounts of rain recently it could lead to more slips," MetService Meteorologist Stephen Glassey warns.
Orange Heavy Rain Warnings and Watches are likely to be issued for the West Coast, Tasman, Nelson and Marlborough regions in the coming days, Met Service predicts.
Other parts of the country will feel like spring as warmer north to northwest winds settle in ahead of the front.
This will be coupled with mostly fine weather as well as elsewhere, with the eastern South Island expected to stay mostly dry, forecasts Met Service, and the rain-bearing front is not expected to reach the North Island until the weekend.
Auckland will be generally fine throughout the week, with a few scattered showers around. Today is expected to be settled, apart from the odd shower north of Whangaparaoa and light southeasterly wind.