A heavy rain warning has been issued for much of the South Island, including Nelson.
MetService forecasts 150 - 250mm rain about the ranges west of Motueka but 100 -150mm elsewhere, in the 18 hour period from 11am Wednesday to 5am Thursday.
Heavy rain is also forecast for Mount Taranaki, with heavy falls expected from late Wednesday morning.
Heavy rain is expected to spread to the Tararua Ranges, Marlborough, Buller, Westland and the Canterbury high country on Wednesday afternoon and to develop in Otago overnight Wednesday.
The rain should ease from about Buller and Marlborough northwards Thursday morning and in Otago and Westland Thursday afternoon or evening.
MetService says rivers and streams in the affected areas may rise rapidly, and slips and flooding are possible.