Areas of the South Island should be prepared for some wild weather in the coming days.

Residents in parts of the country have been warned to brace for widespread flooding, slips and fast-rising rivers as the mid-week weather system hits.

The weather bomb is forecast to move on to the lower South Island late today, then move on to the rest of the country.

The warning criteria is 100mm of rainfall in 24 hours, but some exposed areas could get up to 300mm in 24 hours.