The MetService is forecasting gale-force winds and heavy rain in Southland and Fiordland from early on Friday.
It is predicting severe north-westerly gales, with gusts of up to 130km/h in inland Southland.
Southland Civil Defence is urging householders to check that all their property is secure and they have torches in case of power cuts.
It is also warning it may be dangerous to go outside amongst flying debris if winds reach the strength that the MetService is predicting.
Up to 250 millimetres of rain is expected to fall in Fiordland from Thursday until Friday night.
Organisers of the Tour of Southland cycling race have been warned about the conditions.
