Bearer of the bad weather, carrying gale winds and rain, was a "very active" front preceded by a strong moist northwest flow.
Forecasters said the front would move on from Fiordland and move over Westland this morning with a burst of heavy rain and thunderstorms.
Later in the day it would weaken as it moved north to reach the lower North Island during the evening.
The MetService predicted 100mm to 180mm of rain in parts of Westland with heavy spillover for the headwaters of Canterbury and Otago lakes and rivers of 80mmto 100mm.
As much as 230mm was possible in the Alps near the glaciers.
"This is a significant amount with the bulk falling in 12 or fewer hours, leading to rapidly rising streams and rivers.
Forecasters added severe gale gusts of 130kmh to 140kmh might batter eastern Otago from north of Dunedin, and inland Canterbury.
Such winds had the potential to damage trees and power lines and make driving dangerous.
