Low pressure over the Tasman Sea will bring rain to much of New Zealand during the second half of the weekend, MetService spokesman Daniel Corbett said.

"The approaching low will have a lot of available moisture from the sub-tropics, which could make it quite a wet end to the weekend in many places."

First in line for a drenching is the West Coast of the South Island on Saturday night, and across the rest of the country during Sunday, Mr Corbett said.

"There could be some heavy bursts of rain, especially in northern and western areas from Nelson over to Northland."

MetService had already issued a severe weather warning for Nelson and a watch for many districts further north as well as eastern parts of the South Island. The Otago Regional Council is on flood watch with heavy rain expected on Sunday through to Tuesday.

The heaviest falls were expected on Sunday evening in the Shag and Kakanui catchments.

MetService expected to upgrade its watch to a warning closer to Sunday when details of rainfall amounts become clearer.

The Council advised farmers to be wary of rising river levels and be prepared to move stock and equipment to higher ground where necessary.

Mr Corbett said by Monday the main band rain would be settled along the east coast of the South Island.

The rain would weaken during the middle of next week, followed by a cooler southerly flow, Mr Corbett said.

- APNZ