Fiji's Meteorological Service says hurricane-force winds from Cyclone Gita may gust to 295 km/h as the storm reaches the southern islands.
It's expected to strengthen to a category five today as it moves towards Fiji tonight. The last time a category five cyclone hit the Pacific was in February 2016 when Cyclone Winston killed 44 people in Fiji.
Gita earlier hit the islands of Tongatapu and 'Eua as a category four storm, with winds of up to 230km/h.
A hurricane warning remains in force for Fiji's southern Lau group. Ono-i-Lau and Vatoa are expected to be hit with average wind speeds of up to 195km/h, gusting up to 275km/h, Fiji's Meteorological Service director Ravid Kumar said.
Mr Kumar said a a storm warning was also in force for the remaining islands in the southern Lau Group.
He also said there was a gale warning in force for Matuku, Totoya, Moala, Kadavu and the nearby smaller islands of Lakeba and Nayau.
Gita could remain at Category 5 for up to 48 hours bringing extremely destructive winds, Mr Kumar said.
"They are destructive hurricane force winds with average speeds of 195km/hr and momentary gusts up to 275km/hr close to the centre. as we anticipate it to further intensify the winds are expected to increase to 215km/hr and momentary gusts to 295km/hr."
There is a strong wind warning remaining in force for the rest of Fiji, Mr Kumar said.
Residents of the southern Lau group have been told to evacuate by local civil defence as Cyclone Gita heads their way.
Director General for Red Cross Fiji, Filipe Nainoca, said evacuations had taken place, for fear of an expected sea surge.
The cyclone was slow-moving but residents were expecting to face the brunt of it overnight, Mr Nainoca said.
NDMO said main urban areas like Suva face a lower level threat from Cyclone Gita but people should be taking precautionary measures and still prepare for gale force winds.
NDMO director Anare Leweniqila said the cyclone put the 2500 people living on Ono-I-Lau and Vatoa at risk.
"Those two islands are likely to be facing the cyclone, the [category] 5, the full brunt of the [category] 5 winds."
Preparations were well underway with evacuation centres identified and food and water supplies being gathered, he said.
Huge storm surges were anticipated and the three Ono-i-Lau evacuation centres were located inland from the coast.
The islands are set to face periods of rain and squally thunderstorms, and damaging heavy swells with sea flooding of coastal areas.
Fiji's Meteorological Service warned destructive winds were likely to begin several hours before the cyclone passes overhead or nearby.