Heavy rain could cause slips and flooding in Westland and Fiordland today into tomorrow, MetService says.

It said a series of fronts embedded in a moist northwest flow were expected to move up the South Island.

It could bring up to 200mm of rain to Westland south of Otiri, and also cause heavy falls and possible thunderstorms in Fiordland.

A watch is in place for the headwaters of Canterbury and Otago rivers and lakes.