An intense front over the Tasman Sea is expected to move onto the west of the South Island Saturday morning.

During Saturday this front is forecast to move slowly east over the South Island, preceded by a strong moist northerly flow.

A period of heavy rain is likely for the West Coast and Alps. The heaviest falls are expected in Fiordland north of Milford Sound and in Westland south of Otira, where 150 to 200mm of rain may fall.

Significant spillover rain is also expected for the headwaters of the Canterbury lakes and rivers.

Behind this front, further rain or heavy showers are likely but at lesser intensities. People in these areas are advised rivers and streams may rise rapidly and surface flooding and slips are possible.