A spokesman said an active front was forecast to move northwards across the South Island, delivering a period of heavy rain to the west of the island, and strong northwesterly gales to eastern regions.
The heaviest rain was expected in Westland (south of Otira), Fiordland and the headwaters of the Otago and Canterbury lakes and rivers.
A warning was now in force for these areas, he said.
In the Otago headwaters, between 90mm and 120mm of rain was expected to accumulate near the Main Divide this morning and this afternoon, and 60mm to 80mm within 15km east of the divide.
Peak intensities of 15mm to 25mm per hour would fall, especially near the divide.
In Fiordland, from dawn today until mid-afternoon, between 60mm and 90mm of rain was expected to accumulate, with peak intensities of 15mm to 25mm per hour.
The spokesman said the heavy rain could cause streams and rivers to rise rapidly.
Surface flooding and slips were also possible, and driving conditions may become hazardous.
He advised residents to keep up to date with the latest forecasts, in case other areas were added to the MetService warning.
