Civil Defence West Coast is warning people to prepare for possible flooding and landslides, particularly in Westland.
The MetService issued a heavy rain warning for the south and west of the South Island.
The heaviest rain is expected to fall in Westland, south of Otira, where up to 300 millimeters could fall around the ranges, and up to 150mm in the wider area from the early hours of Thursday morning until 6pm.
Thunderstorms are also possible.
Civil Defence West Coast said people should look out for flooding and landslides, especially if they are travelling.
Coastal communities are also warned to be prepared for possible storm surges, with high tides around midnight tonight and midday tomorrow.