Wellington, March 21 NZPA - Forecasters have issued a severe weather warning as "significant" rain is predicted to fall in parts of the South Island tonight and tomorrow.

A slow moving front over Westland is expected to intensify today, ahead of another front approaching Fiordland.

The second front will bring heavy rain throughout Westland this afternoon and this evening, and Fiordland overnight.

Thunderstorms could drop as much as 140-180mm on the West Coast and around 280mm in the Southern Alps near the glaciers, creating a "spillover" for Canterbury and Otago lakes and rivers.

"This is a significant amount of rain, with the bulk of it falling in 12 hours or less, leading to rapidly rising rivers and streams," Metservice said.

Northwest gales are also forecast to reach 120-130kmh on the Otago coast north of Dunedin and in inland parts of Canterbury.

Driving may be hazardous, especially for motorcycles and high-sided vehicles, Metservice warned.