Emergency Management Southland is warning people in all coastal areas in Southland, including Invercargill, to prepare for severe gales and power cuts on Monday night.
The MetService says severe thunderstorms will brings winds of more than 110 km per hour, rainfall of up to 25mm an hour and could spawn damaging tornadoes.
It says the front will move across the entire South Island in the next day.
The Southland Harbour Master says 6m swells are forecast for Foveaux Strait, and boats in Bluff Harbour could be affected.
The Transport Agency has closed State Highway 94 between Te Anau and Milford Sound for the night because of snow.
Civil defence says loose roofing iron should be secured, outdoor furniture and trampolines tied down and households should have battery-powered torches and radios ready. Boat owners are advised to check their moorings.