Fiji's meteorological office is warning of the high likelihood of a cyclone developing to the north of the group and moving on to the country by Friday.
Current forecast models from the office have the system tracking either over Vanua Levu or west of Viti Levu.
The weather for Christmas is expected to be fine but will deteriorate closer to the weekend.
Rotuma can expect heavy rain by Boxing Day and as the system moves closer to Fiji damaging winds, heavy rain leading to flooding and storm surges may develop.
The low pressure system had a "high chance" of developing into a tropical cyclone.
Members of the public were advised to "remain alert and vigilant".
"The situation is closely monitored and any alert and warning will be issued as and when significant changes are anticipated," it said in a statement.