Cyclone Ella has strengthened to a category 2 storm with gusts of up to 130 km/h, Fiji's Metservice says.
The Fiji Meteorological Service said the cyclone was 480km northeast of Vanua Belavu, the third-largest island in Fiji's Lau archipelago, at 6pm on Wednesday and was moving west southwest towards Fiji at 12km/h.
In a warning issued at 8pm, the Metservice said a tropical cyclone alert remained in force for Lau and the Lamaiviti group, Vanua Levu, Tavenui and nearby smaller islands.
It said people in those areas could expect strong southeasterly winds with average speeds of 45 km/h gusting to 80km/h, which were predicted to increase further to damaging gale force winds of 85km/h to 110km/h from Thursday evening.
For the rest of Fiji, moderate to fresh southeasterly winds were predicted to strengthen with gusts increasing to 80km/h on Friday.
Isolated showers and thunderstorms were also forecast with showers becoming frequent and heavy.
Fiji's Chief of Operations Rusiate Tudravu told FBC News people needed to take the necessary precautions for heavy rain and flash flooding in low lying areas.
People in Tonga's northernmost island group, the Niuas, were also preparing for Cyclone Ella.
The country's National Emergency Management Office director Leveni 'Aho said emergency services were on standby there, where up to 700 people live.
He said locals were preparing for tonight, when the storm would be at its closest.
"Get their fuel and batteries and radio and things like that. The normal things we do in preparation for cyclones."
There were no reports of damage yet, he said.
In Samoa, a heavy rain warning was in effect on Wednesday after the country's weather authorities warned flooding was possible in vulnerable areas.
A Samoan Disaster Management Office spokesperson said landslides were possible and people living on land that could be affected should consider moving to safer areas.
Meanwhile, as Cyclone Donna passed New Caledonia's Loyalty Islands further east than forecast, alerts were lifted and many schools had reopened.