MetService is warning heavy rain and thunderstorms are expected to move slowly over the western side of the country, with Taranaki bearing the brunt of the wet weather.
The rain is forecast to last through Sunday and Monday.
In Fiordland and Westland, heavy rain is expected to last until late Monday morning.
MetService says heavy rain is due to hit northern Wellington, the Kapiti Coast and the Tararua Range on Monday.
The combination of the heavy rain and squally thunderstorms could cause rivers to rise rapidly, flooding and slips.