MetService is forecasting heavy rain for Clutha and Southland tonight.
The forecaster has issued a heavy rain watch, saying it is expected to hit the area between 8pm on Sunday and 8am on Monday.
A MetService spokesman said rainfall amounts may approach warning criteria, and may cause streams and rivers to rise rapidly.
Surface flooding and slips were also possible and driving conditions may be hazardous.
Residents were advised to keep up to date with forecasts in case parts of the forecast were upgraded to "a full warning", or further areas were added.
The forecast comes as a heatwave settles in over much of the rest of the country today. It is expected to bring a run of high temperatures to most of New Zealand over the next six days.
