The Fiji meteorological service says people living in flood prone areas should prepare after a low pressure system near Fiji developed into a tropical disturbance.
Director Meteorology Ravin Kumar said some centres had recorded 80 millimetres of rain in the last 48 hours.
"The ground is now saturated so any rain will flow into the rivers and flooding is possible," he said.
"We remind all communities to take early action. Prepare yourselves if you're living in low lying areas."
The weather office confirmed Tropical Disturbance TD04F was located about 65 kilometres South-Southwest of Rotuma or about 560 kilometres North of Nadi at 6 this morning local time.
A heavy rain alert is currently in force for the whole country, which is still reeling from Cyclone Winston in February with hundreds of people still living in tents.
TD04F is currently slow moving and an associated trough of low pressure affects Fiji.
The system is slowly deepening and is expected to become a Tropical Depression today, and move Southeast towards the Fiji Group.
At this stage, TD04F has a low to moderate or 20 to 50 per cent chance of developing into a tropical cyclone over Fiji waters.