The Metservice in Fiji says a low pressure system in the north could deepen and form a cyclone by Friday.
Its Director of Meteorology, Rajendra Prasad says the system is bringing heavy rain to the northern islands as it passes Vanua Levu later today.
Mr Prasad warns that the weather is set to worsen tomorrow.
"We have this trough located just north of the country. It is moving down on to Vanua Levu and other parts of the group. And already causing rain over the northern, northwestern areas and this rain band is expected to intensify with strong winds which are already affecting sea areas [and may] spread on to land areas."
Rajendra Prasad is advising people heading out to sea to be cautious as high winds are expected.
Mr Prasad says the bad weather may ease by mid-next week.