Heavy rain in Fiordland and Westland in the South Island could result in flooding and slips.
MetService has issued a severe weather warning for Fiordland and parts of Westland for the next two days.
The forecaster says 80 to 120 millimetres of rain is set to fall in Westland, south of Otira, between 7pm tonight and 10am tomorrow morning.
In Fiordland 70 to 100mm of rain could fall between 4pm this afternoon and 4am tomorrow morning.
The forecaster says the heavy rain may cause streams and rivers to rise rapidly, and driving conditions may be hazardous.
There's also a high risk of thunderstorms in Fiordland and western Southland this evening and overnight.
Overnight, there's a moderate to low risk of thunderstorms for wider Southland, parts of inland Otago and Westland.
A front - preceded by a strong and moist northwesterly flow - moving onto Fiordland tonight and across the South Island during Thursday is responsible for the predicted rainfall.
MetService has also issued a severe weather watch for heavy rain in Buller and northern Westland on Thursday as the front moves up the country.
The watch is valid for 6am to 6pm on Thursday, when heavy rain and thunderstorms are possible.
The front is expected to weaken over central New Zealand on Thursday night.