An orange rain warning has been issued for Fiordland north of Doubtful Sound from Wednesday night.
MetService says a front is forecast to move out of the Tasman Sea and across the lower South Island overnight Wednesday and Thursday, preceded by a period of heavy rain for western regions and northwest gales.
Heavy rain could cause streams and rivers to rise rapidly, surface flooding and slips.
The warning is in place from 11pm Wednesday to 11am Thursday.
MetService said 70 to 100mm of rain was expected with peak intensities of 15 to 25 mm per hour Thursday morning.