West Coast Civil Defence Emergency Management spokeswoman Claire Brown said the organisation was taking the forecast seriously and was keeping a close eye on it, after severe weather battered the region late last month, causing widespread flooding.
She asked residents to visit www.westcoastemergency.govt.nz for updated information on affected areas.
A moist northerly flow strengthened over the South Island yesterday, ahead of an active front forecast to move across the island from the west today, a MetService spokesman 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."
He said the heavy rain may cause streams and rivers to rise rapidly.
"Surface flooding and slips are also possible and driving conditions may be hazardous."
Between 150mm and 200mm of rain was forecast to fall in Westland, south of Otira, and 80mm to 100mm was forecast to fall elsewhere until 9pm tonight.
"Peak intensities of 20mm to 30mm per hour and thunderstorms are possible.
"There is also a slight risk of a small coastal tornado."
In Fiordland, north of Doubtful Sound, 90mm to 120mm of rain is forecast to accumulate until 1pm today, and peak intensities of 20mm to 30mm per hour and thunderstorms are possible.
The active front was also expected to deliver a period of heavy northerly rain to the headwaters of the Otago lakes and rivers until 3pm today.
"Rainfall amounts may approach warning criteria within 15km east of the Main Divide," he said.
