The National Weather Forecasting Centre in Nadi states a trough of low pressure is expected to develop and move over Fiji from the west starting later today.
The system is forecast to bring widespread cloud cover, showers, and periods of rain across much of the country through to late Wednesday.
Areas expected to be most affected include the Northern Division, Lau and Lomaiviti groups, Serua-Namosi, Navua-Suva-Nausori, Tailevu-Naitasiri-Ra, Kadavu, and nearby smaller islands.
The forecast includes occasional showers and possible thunderstorms, with conditions expected to worsen from later Tuesday, as showers become more frequent and increase to steady rain.
The weather office is also warning of localised heavy rainfall, which may lead to flash flooding in low-lying and flood-prone areas.