An extensive and deepening low over the Tasman Sea is expected to approach the upper South Island from the northwest today. The low should move southeast across the South Island Thursday morning.

A humid northerly flow ahead of the low is expected to bring rain to much of the country. Rain is expected to become heavy in Nelson, where 250mm of rain may fall in the west, and about Mt Taranaki late Wednesday morning. Heavy rain is expected to spread to the Tararua Ranges, Marlborough, Buller, Westland and the Canterbury high country Wednesday afternoon. Heavy rain is forecast to develop in Otago overnight Wednesday. The rain should ease from about Buller and Marlborough northwards Thursday morning and in Otago and Westland Thursday afternoon or evening.

People in these areas are advised rivers and streams may rise rapidly, slips and surface flooding are possible and driving conditions could become hazardous.