Tropical Cyclone Ula is crossing Fiji's southern Lau group and may intensify.
The small islands of Kabara, Namuka, Fulaga, Ogea, Vatoa and Ono-i-Lau are warned to expect destructive storm force winds of up to 155 kilometres an hour.
The category three system, which at its centres has average winds of 150 kilometres an hour and gusts of more than 200 kilometres and hour, is tracking southwest between the islands of Kabara and Ono-i-Lau.
The cyclone has slowed and the Fiji Met Service says it may intensify.
By about seven o'clock tonight Cyclone Ula is forecast to get within about 65 kilometres of Ono-i-Lau.
Fiji's education ministry has advised all school evacuation centres to be on standby for immediate activation.
Hurricane force winds and phenomenal seas are forecast for the area within 25 kilometres of Ula's centre.
Tonga's state of emergency issued for Vava'u and Ha'apai remains in force.
The Deputy Prime Minister, Siaosi Sovaleni, who is the Chairman of the National Emergency Management Committee, says Ula caused no casualties.
He says almost 400 people had been moved to evacuation centres to protect them against the storm.
The director of Tonga Met, 'Ofa Fa'anunu, says forecasters were surprised that Ula sustained its strength.
He says had there been more moisture, the cyclone could have developed into a category five system.