Parts of the South Island are bracing for flooding and slips, with a state of emergency in place on the West Coast and 'red' severe weather warnings for Buller and Westland.

Rain started on the West Coast overnight and is expected to get heavier through the day, causing flooding and dangerous river conditions.

Civil Defence urges Westport residents whose properties have previously flooded to keep a bag packed as they may have to evacuate their homes.

A state of emergency has been declared after severe weather warnings were raised to red for Buller and Westland until Thursday afternoon.

Further north, emergency operations are set to ramp up at the top of the South Island, with orange level warnings in place in the Tasman, Marlborough and Nelson regions.

Slips, rising rivers and surface flooding were expected to cause disruption in these parts.

MetService says up to 800mm of rain could accumulate in the Tasman ranges until Thursday.

Tasman Civil Defence controller Rob Smith says another downpour lashing the already waterlogged region could present problems.

Waka Kotahi urges motorists to delay non-essential travel until Thursday as road closures are expected.

An "exceptional" atmospheric river is bringing the heavy rain to the west and north of the South Island, NIWA meteorologist Tristan Meyers says.

The large plume of moisture thousands of kilometres long is stretching from the tropics and making its way to New Zealand.

"We're in the midst of one particularly intense one right now - we usually don't get moisture like this in the atmosphere in our neck of the woods in wintertime ... so it's really quite exceptional."

-RNZ.