"This warning is for a period of heavy rain in Fiordland, Westland, the headwaters of Otago and Canterbury Lakes and Rivers from Arthurs Pass southwards, Buller and the Tararua Range," said MetService meteorologist Andy Best.
"In addition ... a period of severe gale northwesterlies in Fiordland, Southland including Stewart Island, Otago, Canterbury, Marlborough, Wellington and Wairarapa including the Tararua District."
Fiordland is expected to get rain of up to 200mm by 9pm this evening. Northwest winds of up to 95kmh was expected in the Cook Strait by 1am Monday.
WeatherWatch says most of northern New Zealand will enjoy lots of sunshine today, but the cold front moving in means the high will slip away.
"The low will move eastward on Monday, and the main areas of heavy rain will change into the southern parts of the North Island," WeatherWatch analyst Phil Duncan said.
"About the south island, rain will also continue across the north part on Monday."
But by Tuesday, sunshine will return to both islands as high pressure in the Tasman sea cover the country.