An "extended period of significant heavy rain" is forecast for the northwest areas of the South Island warns MetService.
The weather system is expected to arrive on Monday, moving eastwards across the South Island before leaving on Thursday evening.
The Tasman District, Buller and Westland can expect 63-66 hours of heavy rainfall causing streams and rivers to rise rapidly, creating surface flooding and slips.
Motorists are warned that driving may be dangerous.
MetService warns "this is likely to be a significant event, especially for places where soils are already saturated from recent rain."
"People are advised to keep up to date with the latest forecasts, as some areas could be upgraded to a Red Warning."