Two weather fronts converging in the South Island are likely to dump up to 120mm of rain on Westland, south of the glaciers, on Wednesday.
Residents and especially trampers are warned to look out for rapidly rising rivers and slips as rain intensifies.
About 15mm to 20mm per hour are forecast to fall from Wednesday evening through to Thursday morning, MetService says.
Freezing levels are expected to be above 2200m in Westland.
Heavy rain, wind to hit the north
Northland and Auckland residents may also be battered by gales and heavy rain later this week as forecasters keep their eye on a low moving in from the Tasman Sea.
The system's front is expected to hit Northland by Wednesday afternoon through to Thursday morning and Auckland overnight on Wednesday, MetService says.
There is a possibility it could bring heavy rainfall approaching warning levels of at least 85mm within 18 hours.