The MetService issued a severe thunderstorm watch for Wanganui, Feilding and Mangaweka on Sunday.
These areas have already been affected by thunderstorms and heavy rain.
The MetService predicts more thunderstorms are likely until 8pm on Sunday, while over the same time some areas are likely to receive up to 40 millimetres of rain.
It warns that heavy rain may cause flash flooding, especially in low-lying areas.
Motorists in Wanganui and Manawatu are advised to drive carefully as poor visibility and surface flooding are possible.