Heavy rain and flood warnings are in effect for parts of Fiordland and Westland that were hit by floods last month.
MetService is predicting heavy rain in southern Westland and northern Fiordland today and warns that streams and rivers could rise rapidly.
Surface flooding and slips are possible and driving conditions could be dangerous.
In Westland south of Otira, 150 to 200mm of rain is expected in the ranges, with 80 to 100mm in lower areas.
In Fiordland north of Doubtful Sound, 90 to 120mm of rain is expected.
As much as 20mm to 30mm of rain could fall in an hour and thunderstorms are possible in Westland and Fiordland.
"A moist northerly flow strengthens over the South Island today ahead of an active front forecast to move across the island from the west on Wednesday," MetService said.
"This front delivers a period of heavy northerly rain to western parts of the South Island, with the heaviest falls expected in Fiordland north of Doubtful Sound and Westland south of Otira, where a Heavy Rain Warning is now in force."
People are advised to stay up to date with the latest forecasts in case new areas are added to the warning.
Heavy rain caused severe flooding late last month, washing out parts of an old landfill near Fox Glacier and damaging Waiho Bridge, closing access between Franz Josef and Fox townships on State Highway 6.