Weather: 'Unusually high moisture' to fuel heavy rain in South Island

Heavy rain is on the way for the South Island as settled weather makes way for a "strong northerly flow".

MetService says that flow is developing ahead of a front which will arrive hit the South Island overnight Wednesday.

"Unusually high moisture" will act as a fuel for heavy rain on the West Coast of the South Island later this week, forecaster NIWA says.

A heavy rain warning is in place for Westland south of Otira, with between 250 and 350mm of rain expected from 3pm Thursday to 11pm Friday.

"Heavy rain may cause rivers and streams to rise rapidly," MetService says on its website.

"Surface flooding and slips are also possible and driving conditions may be hazardous."

WeatherWatch is also forecasting rain with some heavy downpours.

"The West Coast has some rain later this week and a burst of strong west to northwest winds up to about the lower North Island," the forecaster says.